diff --git a/src/PhpSpreadsheet/Calculation/Database.php b/src/PhpSpreadsheet/Calculation/Database.php
index 2ba4af2d..5250e306 100644
--- a/src/PhpSpreadsheet/Calculation/Database.php
+++ b/src/PhpSpreadsheet/Calculation/Database.php
@@ -33,7 +33,7 @@ class Database
         }
         $key = array_search($field, $fieldNames);
 
-        return ($key) ? $key : null;
+        return $key ?: null;
     }
 
     /**
diff --git a/src/PhpSpreadsheet/Calculation/Financial.php b/src/PhpSpreadsheet/Calculation/Financial.php
index 3c24eb3b..f0b5ab05 100644
--- a/src/PhpSpreadsheet/Calculation/Financial.php
+++ b/src/PhpSpreadsheet/Calculation/Financial.php
@@ -2270,7 +2270,7 @@ class Financial
         // create an initial range, with a root somewhere between 0 and guess
         $guess = Functions::flattenSingleValue($guess);
         $x1 = 0.0;
-        $x2 = $guess ? $guess : 0.1;
+        $x2 = $guess ?: 0.1;
         $f1 = self::xnpvOrdered($x1, $values, $dates, false);
         $f2 = self::xnpvOrdered($x2, $values, $dates, false);
         $found = false;
@@ -2306,12 +2306,12 @@ class Financial
      *
      * @param float $rate the discount rate to apply to the cash flows
      * @param float[] $values     A series of cash flows that corresponds to a schedule of payments in dates.
-     *                                         The first payment is optional and corresponds to a cost or payment that occurs at the beginning of the investment.
-     *                                         If the first value is a cost or payment, it must be a negative value. All succeeding payments are discounted based on a 365-day year.
-     *                                         The series of values must contain at least one positive value and one negative value.
+     *                                 The first payment is optional and corresponds to a cost or payment that occurs at the beginning of the investment.
+     *                                 If the first value is a cost or payment, it must be a negative value. All succeeding payments are discounted based on a 365-day year.
+     *                                 The series of values must contain at least one positive value and one negative value.
      * @param mixed[] $dates      A schedule of payment dates that corresponds to the cash flow payments.
-     *                                         The first payment date indicates the beginning of the schedule of payments.
-     *                                         All other dates must be later than this date, but they may occur in any order.
+     *                                 The first payment date indicates the beginning of the schedule of payments.
+     *                                 All other dates must be later than this date, but they may occur in any order.
      *
      * @return float|mixed|string
      */
diff --git a/src/PhpSpreadsheet/Calculation/MathTrig.php b/src/PhpSpreadsheet/Calculation/MathTrig.php
index 0ba109f3..e62e9497 100644
--- a/src/PhpSpreadsheet/Calculation/MathTrig.php
+++ b/src/PhpSpreadsheet/Calculation/MathTrig.php
@@ -146,8 +146,8 @@ class MathTrig
         $xCoordinate = Functions::flattenSingleValue($xCoordinate);
         $yCoordinate = Functions::flattenSingleValue($yCoordinate);
 
-        $xCoordinate = ($xCoordinate !== null) ? $xCoordinate : 0.0;
-        $yCoordinate = ($yCoordinate !== null) ? $yCoordinate : 0.0;
+        $xCoordinate = $xCoordinate ?? 0.0;
+        $yCoordinate = $yCoordinate ?? 0.0;
 
         if (
             ((is_numeric($xCoordinate)) || (is_bool($xCoordinate))) &&
diff --git a/src/PhpSpreadsheet/Chart/Axis.php b/src/PhpSpreadsheet/Chart/Axis.php
index 7995c3b3..455a5faa 100644
--- a/src/PhpSpreadsheet/Chart/Axis.php
+++ b/src/PhpSpreadsheet/Chart/Axis.php
@@ -340,9 +340,9 @@ class Axis extends Properties
     {
         $this->setShadowPresetsProperties((int) $sh_presets)
             ->setShadowColor(
-                $sh_color_value === null ? $this->shadowProperties['color']['value'] : $sh_color_value,
-                $sh_color_alpha === null ? (int) $this->shadowProperties['color']['alpha'] : $sh_color_alpha,
-                $sh_color_type === null ? $this->shadowProperties['color']['type'] : $sh_color_type
+                $sh_color_value ?? $this->shadowProperties['color']['value'],
+                $sh_color_alpha ?? (int) $this->shadowProperties['color']['alpha'],
+                $sh_color_type ?? $this->shadowProperties['color']['type']
             )
             ->setShadowBlur($sh_blur)
             ->setShadowAngle($sh_angle)
@@ -482,9 +482,9 @@ class Axis extends Properties
     {
         $this->setGlowSize($size)
             ->setGlowColor(
-                $color_value === null ? $this->glowProperties['color']['value'] : $color_value,
-                $color_alpha === null ? (int) $this->glowProperties['color']['alpha'] : $color_alpha,
-                $color_type === null ? $this->glowProperties['color']['type'] : $color_type
+                $color_value ?? $this->glowProperties['color']['value'],
+                $color_alpha ?? (int) $this->glowProperties['color']['alpha'],
+                $color_type ?? $this->glowProperties['color']['type']
             );
     }
 
diff --git a/src/PhpSpreadsheet/Chart/GridLines.php b/src/PhpSpreadsheet/Chart/GridLines.php
index 2e424bc2..385b278b 100644
--- a/src/PhpSpreadsheet/Chart/GridLines.php
+++ b/src/PhpSpreadsheet/Chart/GridLines.php
@@ -291,9 +291,9 @@ class GridLines extends Properties
         $this->activateObject()
             ->setShadowPresetsProperties((int) $sh_presets)
             ->setShadowColor(
-                $sh_color_value === null ? $this->shadowProperties['color']['value'] : $sh_color_value,
+                $sh_color_value ?? $this->shadowProperties['color']['value'],
                 $sh_color_alpha === null ? (int) $this->shadowProperties['color']['alpha'] : $this->getTrueAlpha($sh_color_alpha),
-                $sh_color_type === null ? $this->shadowProperties['color']['type'] : $sh_color_type
+                $sh_color_type ?? $this->shadowProperties['color']['type']
             )
             ->setShadowBlur($sh_blur)
             ->setShadowAngle($sh_angle)
diff --git a/src/PhpSpreadsheet/Helper/Html.php b/src/PhpSpreadsheet/Helper/Html.php
index 6c4cbf9b..f07bc961 100644
--- a/src/PhpSpreadsheet/Helper/Html.php
+++ b/src/PhpSpreadsheet/Helper/Html.php
@@ -711,7 +711,7 @@ class Html
                 } elseif (strpos(trim($attributeValue), '#') === 0) {
                     $this->$attributeName = ltrim($attributeValue, '#');
                 } else {
-                    $this->$attributeName = $this->colourNameLookup($attributeValue);
+                    $this->$attributeName = static::colourNameLookup($attributeValue);
                 }
             } else {
                 $this->$attributeName = $attributeValue;
diff --git a/src/PhpSpreadsheet/Reader/Gnumeric.php b/src/PhpSpreadsheet/Reader/Gnumeric.php
index dc921c1e..2bec2a13 100644
--- a/src/PhpSpreadsheet/Reader/Gnumeric.php
+++ b/src/PhpSpreadsheet/Reader/Gnumeric.php
@@ -657,7 +657,7 @@ class Gnumeric extends BaseReader
         $column = $columnAttributes['No'];
         $columnWidth = ((float) $columnAttributes['Unit']) / 5.4;
         $hidden = (isset($columnAttributes['Hidden'])) && ((string) $columnAttributes['Hidden'] == '1');
-        $columnCount = (isset($columnAttributes['Count'])) ? $columnAttributes['Count'] : 1;
+        $columnCount = $columnAttributes['Count'] ?? 1;
         while ($c < $column) {
             $this->spreadsheet->getActiveSheet()->getColumnDimension(Coordinate::stringFromColumnIndex($c + 1))->setWidth($defaultWidth);
             ++$c;
@@ -696,7 +696,7 @@ class Gnumeric extends BaseReader
         $row = $rowAttributes['No'];
         $rowHeight = (float) $rowAttributes['Unit'];
         $hidden = (isset($rowAttributes['Hidden'])) && ((string) $rowAttributes['Hidden'] == '1');
-        $rowCount = (isset($rowAttributes['Count'])) ? $rowAttributes['Count'] : 1;
+        $rowCount = $rowAttributes['Count'] ?? 1;
         while ($r < $row) {
             ++$r;
             $this->spreadsheet->getActiveSheet()->getRowDimension($r)->setRowHeight($defaultHeight);
diff --git a/src/PhpSpreadsheet/Reader/Slk.php b/src/PhpSpreadsheet/Reader/Slk.php
index e58ff2f6..89d80ffa 100644
--- a/src/PhpSpreadsheet/Reader/Slk.php
+++ b/src/PhpSpreadsheet/Reader/Slk.php
@@ -419,14 +419,14 @@ class Slk extends BaseReader
         if ($columnWidth > '') {
             if ($startCol == $endCol) {
                 $startCol = Coordinate::stringFromColumnIndex((int) $startCol);
-                $spreadsheet->getActiveSheet()->getColumnDimension($startCol)->setWidth($columnWidth);
+                $spreadsheet->getActiveSheet()->getColumnDimension($startCol)->setWidth((float) $columnWidth);
             } else {
                 $startCol = Coordinate::stringFromColumnIndex($startCol);
                 $endCol = Coordinate::stringFromColumnIndex($endCol);
                 $spreadsheet->getActiveSheet()->getColumnDimension($startCol)->setWidth((float) $columnWidth);
                 do {
-                    $spreadsheet->getActiveSheet()->getColumnDimension(++$startCol)->setWidth($columnWidth);
-                } while ($startCol != $endCol);
+                    $spreadsheet->getActiveSheet()->getColumnDimension(++$startCol)->setWidth((float) $columnWidth);
+                } while ($startCol !== $endCol);
             }
         }
     }
diff --git a/src/PhpSpreadsheet/Reader/Xls.php b/src/PhpSpreadsheet/Reader/Xls.php
index 5656a0ff..6d6b87fd 100644
--- a/src/PhpSpreadsheet/Reader/Xls.php
+++ b/src/PhpSpreadsheet/Reader/Xls.php
@@ -1703,7 +1703,8 @@ class Xls extends BaseReader
                 //        max 2048 bytes will probably throw a wobbly.
                 $row = self::getUInt2d($recordData, 0);
                 $extension = true;
-                $cellAddress = array_pop(array_keys($this->phpSheet->getComments()));
+                $arrayKeys = array_keys($this->phpSheet->getComments());
+                $cellAddress = array_pop($arrayKeys);
             }
 
             $cellAddress = str_replace('$', '', $cellAddress);
diff --git a/src/PhpSpreadsheet/Reader/Xlsx.php b/src/PhpSpreadsheet/Reader/Xlsx.php
index 2b37896d..bf754591 100644
--- a/src/PhpSpreadsheet/Reader/Xlsx.php
+++ b/src/PhpSpreadsheet/Reader/Xlsx.php
@@ -516,7 +516,7 @@ class Xlsx extends BaseReader
                             }
 
                             $style = (object) [
-                                'numFmt' => $numFmt === null ? NumberFormat::FORMAT_GENERAL : $numFmt,
+                                'numFmt' => $numFmt ?? NumberFormat::FORMAT_GENERAL,
                                 'font' => $xmlStyles->fonts->font[(int) ($xf['fontId'])],
                                 'fill' => $xmlStyles->fills->fill[(int) ($xf['fillId'])],
                                 'border' => $xmlStyles->borders->border[(int) ($xf['borderId'])],
diff --git a/src/PhpSpreadsheet/Reader/Xml.php b/src/PhpSpreadsheet/Reader/Xml.php
index 9de88233..f38a9515 100644
--- a/src/PhpSpreadsheet/Reader/Xml.php
+++ b/src/PhpSpreadsheet/Reader/Xml.php
@@ -421,7 +421,7 @@ class Xml extends BaseReader
         $xml_ss = $xml->children($namespaces['ss']);
 
         foreach ($xml_ss->Worksheet as $worksheetx) {
-            $worksheet = ($worksheetx === null) ? new SimpleXMLElement('<xml></xml>') : $worksheetx;
+            $worksheet = $worksheetx ?? new SimpleXMLElement('<xml></xml>');
             $worksheet_ss = self::getAttributes($worksheet, $namespaces['ss']);
 
             if (
@@ -665,7 +665,7 @@ class Xml extends BaseReader
         foreach ($xml->Styles[0] as $style) {
             $style_ss = self::getAttributes($style, $namespaces['ss']);
             $styleID = (string) $style_ss['ID'];
-            $this->styles[$styleID] = (isset($this->styles['Default'])) ? $this->styles['Default'] : [];
+            $this->styles[$styleID] = $this->styles['Default'] ?? [];
             foreach ($style as $styleType => $styleDatax) {
                 $styleData = $styleDatax ?? new SimpleXMLElement('<xml></xml>');
                 $styleAttributes = $styleData->attributes($namespaces['ss']);
diff --git a/src/PhpSpreadsheet/Style/Style.php b/src/PhpSpreadsheet/Style/Style.php
index f7c1be23..7fec9a00 100644
--- a/src/PhpSpreadsheet/Style/Style.php
+++ b/src/PhpSpreadsheet/Style/Style.php
@@ -359,7 +359,9 @@ class Style extends Supervisor
                         $cellIterator = $columnIterator->getCellIterator();
                         $cellIterator->setIterateOnlyExistingCells(true);
                         foreach ($cellIterator as $columnCell) {
-                            $columnCell->getStyle()->applyFromArray($pStyles);
+                            if ($columnCell !== null) {
+                                $columnCell->getStyle()->applyFromArray($pStyles);
+                            }
                         }
                     }
 
@@ -367,7 +369,7 @@ class Style extends Supervisor
                 case 'ROW':
                     $oldXfIndexes = [];
                     for ($row = $rangeStart[1]; $row <= $rangeEnd[1]; ++$row) {
-                        if ($this->getActiveSheet()->getRowDimension($row)->getXfIndex() == null) {
+                        if ($this->getActiveSheet()->getRowDimension($row)->getXfIndex() === null) {
                             $oldXfIndexes[0] = true; // row without explicit style should be formatted based on default style
                         } else {
                             $oldXfIndexes[$this->getActiveSheet()->getRowDimension($row)->getXfIndex()] = true;
@@ -377,7 +379,9 @@ class Style extends Supervisor
                         $cellIterator = $rowIterator->getCellIterator();
                         $cellIterator->setIterateOnlyExistingCells(true);
                         foreach ($cellIterator as $rowCell) {
-                            $rowCell->getStyle()->applyFromArray($pStyles);
+                            if ($rowCell !== null) {
+                                $rowCell->getStyle()->applyFromArray($pStyles);
+                            }
                         }
                     }
 
@@ -423,8 +427,8 @@ class Style extends Supervisor
                 case 'ROW':
                     for ($row = $rangeStart[1]; $row <= $rangeEnd[1]; ++$row) {
                         $rowDimension = $this->getActiveSheet()->getRowDimension($row);
-                        $oldXfIndex = $rowDimension->getXfIndex() === null ?
-                            0 : $rowDimension->getXfIndex(); // row without explicit style should be formatted based on default style
+                        // row without explicit style should be formatted based on default style
+                        $oldXfIndex = $rowDimension->getXfIndex() ?? 0;
                         $rowDimension->setXfIndex($newXfIndexes[$oldXfIndex]);
                     }
 
diff --git a/src/PhpSpreadsheet/Worksheet/CellIterator.php b/src/PhpSpreadsheet/Worksheet/CellIterator.php
index 45f76cab..e0f35b64 100644
--- a/src/PhpSpreadsheet/Worksheet/CellIterator.php
+++ b/src/PhpSpreadsheet/Worksheet/CellIterator.php
@@ -30,10 +30,8 @@ abstract class CellIterator implements Iterator
 
     /**
      * Get loop only existing cells.
-     *
-     * @return bool
      */
-    public function getIterateOnlyExistingCells()
+    public function getIterateOnlyExistingCells(): bool
     {
         return $this->onlyExistingCells;
     }
@@ -45,10 +43,8 @@ abstract class CellIterator implements Iterator
 
     /**
      * Set the iterator to loop only existing cells.
-     *
-     * @param bool $value
      */
-    public function setIterateOnlyExistingCells($value): void
+    public function setIterateOnlyExistingCells(bool $value): void
     {
         $this->onlyExistingCells = (bool) $value;
 
diff --git a/src/PhpSpreadsheet/Worksheet/Column.php b/src/PhpSpreadsheet/Worksheet/Column.php
index 410e8073..763806a0 100644
--- a/src/PhpSpreadsheet/Worksheet/Column.php
+++ b/src/PhpSpreadsheet/Worksheet/Column.php
@@ -41,10 +41,8 @@ class Column
 
     /**
      * Get column index as string eg: 'A'.
-     *
-     * @return string
      */
-    public function getColumnIndex()
+    public function getColumnIndex(): string
     {
         return $this->columnIndex;
     }
diff --git a/src/PhpSpreadsheet/Worksheet/ColumnCellIterator.php b/src/PhpSpreadsheet/Worksheet/ColumnCellIterator.php
index 714ee7ce..1390214e 100644
--- a/src/PhpSpreadsheet/Worksheet/ColumnCellIterator.php
+++ b/src/PhpSpreadsheet/Worksheet/ColumnCellIterator.php
@@ -2,6 +2,7 @@
 
 namespace PhpOffice\PhpSpreadsheet\Worksheet;
 
+use PhpOffice\PhpSpreadsheet\Cell\Cell;
 use PhpOffice\PhpSpreadsheet\Cell\Coordinate;
 use PhpOffice\PhpSpreadsheet\Exception as PhpSpreadsheetException;
 
@@ -59,7 +60,7 @@ class ColumnCellIterator extends CellIterator
      *
      * @return $this
      */
-    public function resetStart($startRow = 1)
+    public function resetStart(int $startRow = 1)
     {
         $this->startRow = $startRow;
         $this->adjustForExistingOnlyRange();
@@ -77,7 +78,7 @@ class ColumnCellIterator extends CellIterator
      */
     public function resetEnd($endRow = null)
     {
-        $this->endRow = ($endRow) ? $endRow : $this->worksheet->getHighestRow();
+        $this->endRow = $endRow ?: $this->worksheet->getHighestRow();
         $this->adjustForExistingOnlyRange();
 
         return $this;
@@ -90,7 +91,7 @@ class ColumnCellIterator extends CellIterator
      *
      * @return $this
      */
-    public function seek($row = 1)
+    public function seek(int $row = 1)
     {
         if ($this->onlyExistingCells && !($this->worksheet->cellExistsByColumnAndRow($this->columnIndex, $row))) {
             throw new PhpSpreadsheetException('In "IterateOnlyExistingCells" mode and Cell does not exist');
@@ -113,20 +114,16 @@ class ColumnCellIterator extends CellIterator
 
     /**
      * Return the current cell in this worksheet column.
-     *
-     * @return \PhpOffice\PhpSpreadsheet\Cell\Cell
      */
-    public function current()
+    public function current(): ?Cell
     {
         return $this->worksheet->getCellByColumnAndRow($this->columnIndex, $this->currentRow);
     }
 
     /**
      * Return the current iterator key.
-     *
-     * @return int
      */
-    public function key()
+    public function key(): int
     {
         return $this->currentRow;
     }
@@ -161,10 +158,8 @@ class ColumnCellIterator extends CellIterator
 
     /**
      * Indicate if more rows exist in the worksheet range of rows that we're iterating.
-     *
-     * @return bool
      */
-    public function valid()
+    public function valid(): bool
     {
         return $this->currentRow <= $this->endRow && $this->currentRow >= $this->startRow;
     }
diff --git a/src/PhpSpreadsheet/Worksheet/ColumnDimension.php b/src/PhpSpreadsheet/Worksheet/ColumnDimension.php
index 4e87a344..12b1efdf 100644
--- a/src/PhpSpreadsheet/Worksheet/ColumnDimension.php
+++ b/src/PhpSpreadsheet/Worksheet/ColumnDimension.php
@@ -43,10 +43,8 @@ class ColumnDimension extends Dimension
 
     /**
      * Get column index as string eg: 'A'.
-     *
-     * @return string
      */
-    public function getColumnIndex()
+    public function getColumnIndex(): string
     {
         return $this->columnIndex;
     }
@@ -54,23 +52,19 @@ class ColumnDimension extends Dimension
     /**
      * Set column index as string eg: 'A'.
      *
-     * @param string $pValue
-     *
      * @return $this
      */
-    public function setColumnIndex($pValue)
+    public function setColumnIndex(string $index)
     {
-        $this->columnIndex = $pValue;
+        $this->columnIndex = $index;
 
         return $this;
     }
 
     /**
      * Get Width.
-     *
-     * @return float
      */
-    public function getWidth()
+    public function getWidth(): float
     {
         return $this->width;
     }
@@ -78,23 +72,19 @@ class ColumnDimension extends Dimension
     /**
      * Set Width.
      *
-     * @param float $pValue
-     *
      * @return $this
      */
-    public function setWidth($pValue)
+    public function setWidth(float $width)
     {
-        $this->width = $pValue;
+        $this->width = $width;
 
         return $this;
     }
 
     /**
      * Get Auto Size.
-     *
-     * @return bool
      */
-    public function getAutoSize()
+    public function getAutoSize(): bool
     {
         return $this->autoSize;
     }
@@ -102,13 +92,11 @@ class ColumnDimension extends Dimension
     /**
      * Set Auto Size.
      *
-     * @param bool $pValue
-     *
      * @return $this
      */
-    public function setAutoSize($pValue)
+    public function setAutoSize(bool $autosizeEnabled)
     {
-        $this->autoSize = $pValue;
+        $this->autoSize = $autosizeEnabled;
 
         return $this;
     }
diff --git a/src/PhpSpreadsheet/Worksheet/ColumnIterator.php b/src/PhpSpreadsheet/Worksheet/ColumnIterator.php
index d0bb20cc..a7466881 100644
--- a/src/PhpSpreadsheet/Worksheet/ColumnIterator.php
+++ b/src/PhpSpreadsheet/Worksheet/ColumnIterator.php
@@ -67,11 +67,13 @@ class ColumnIterator implements Iterator
      *
      * @return $this
      */
-    public function resetStart($startColumn = 'A')
+    public function resetStart(string $startColumn = 'A')
     {
         $startColumnIndex = Coordinate::columnIndexFromString($startColumn);
         if ($startColumnIndex > Coordinate::columnIndexFromString($this->worksheet->getHighestColumn())) {
-            throw new Exception("Start column ({$startColumn}) is beyond highest column ({$this->worksheet->getHighestColumn()})");
+            throw new Exception(
+                "Start column ({$startColumn}) is beyond highest column ({$this->worksheet->getHighestColumn()})"
+            );
         }
 
         $this->startColumnIndex = $startColumnIndex;
@@ -92,7 +94,7 @@ class ColumnIterator implements Iterator
      */
     public function resetEnd($endColumn = null)
     {
-        $endColumn = $endColumn ? $endColumn : $this->worksheet->getHighestColumn();
+        $endColumn = $endColumn ?: $this->worksheet->getHighestColumn();
         $this->endColumnIndex = Coordinate::columnIndexFromString($endColumn);
 
         return $this;
@@ -105,11 +107,13 @@ class ColumnIterator implements Iterator
      *
      * @return $this
      */
-    public function seek($column = 'A')
+    public function seek(string $column = 'A')
     {
         $column = Coordinate::columnIndexFromString($column);
         if (($column < $this->startColumnIndex) || ($column > $this->endColumnIndex)) {
-            throw new PhpSpreadsheetException("Column $column is out of range ({$this->startColumnIndex} - {$this->endColumnIndex})");
+            throw new PhpSpreadsheetException(
+                "Column $column is out of range ({$this->startColumnIndex} - {$this->endColumnIndex})"
+            );
         }
         $this->currentColumnIndex = $column;
 
@@ -136,10 +140,8 @@ class ColumnIterator implements Iterator
 
     /**
      * Return the current iterator key.
-     *
-     * @return string
      */
-    public function key()
+    public function key(): string
     {
         return Coordinate::stringFromColumnIndex($this->currentColumnIndex);
     }
@@ -162,10 +164,8 @@ class ColumnIterator implements Iterator
 
     /**
      * Indicate if more columns exist in the worksheet range of columns that we're iterating.
-     *
-     * @return bool
      */
-    public function valid()
+    public function valid(): bool
     {
         return $this->currentColumnIndex <= $this->endColumnIndex && $this->currentColumnIndex >= $this->startColumnIndex;
     }
diff --git a/src/PhpSpreadsheet/Worksheet/Dimension.php b/src/PhpSpreadsheet/Worksheet/Dimension.php
index a27daf09..4b3a0da8 100644
--- a/src/PhpSpreadsheet/Worksheet/Dimension.php
+++ b/src/PhpSpreadsheet/Worksheet/Dimension.php
@@ -47,10 +47,8 @@ abstract class Dimension
 
     /**
      * Get Visible.
-     *
-     * @return bool
      */
-    public function getVisible()
+    public function getVisible(): bool
     {
         return $this->visible;
     }
@@ -58,23 +56,19 @@ abstract class Dimension
     /**
      * Set Visible.
      *
-     * @param bool $pValue
-     *
      * @return $this
      */
-    public function setVisible($pValue)
+    public function setVisible(bool $visible)
     {
-        $this->visible = (bool) $pValue;
+        $this->visible = $visible;
 
         return $this;
     }
 
     /**
      * Get Outline Level.
-     *
-     * @return int
      */
-    public function getOutlineLevel()
+    public function getOutlineLevel(): int
     {
         return $this->outlineLevel;
     }
@@ -83,27 +77,23 @@ abstract class Dimension
      * Set Outline Level.
      * Value must be between 0 and 7.
      *
-     * @param int $pValue
-     *
      * @return $this
      */
-    public function setOutlineLevel($pValue)
+    public function setOutlineLevel(int $level)
     {
-        if ($pValue < 0 || $pValue > 7) {
+        if ($level < 0 || $level > 7) {
             throw new PhpSpreadsheetException('Outline level must range between 0 and 7.');
         }
 
-        $this->outlineLevel = $pValue;
+        $this->outlineLevel = $level;
 
         return $this;
     }
 
     /**
      * Get Collapsed.
-     *
-     * @return bool
      */
-    public function getCollapsed()
+    public function getCollapsed(): bool
     {
         return $this->collapsed;
     }
@@ -111,13 +101,11 @@ abstract class Dimension
     /**
      * Set Collapsed.
      *
-     * @param bool $pValue
-     *
      * @return $this
      */
-    public function setCollapsed($pValue)
+    public function setCollapsed(bool $collapsed)
     {
-        $this->collapsed = (bool) $pValue;
+        $this->collapsed = $collapsed;
 
         return $this;
     }
@@ -127,7 +115,7 @@ abstract class Dimension
      *
      * @return int
      */
-    public function getXfIndex()
+    public function getXfIndex(): ?int
     {
         return $this->xfIndex;
     }
@@ -135,11 +123,9 @@ abstract class Dimension
     /**
      * Set index to cellXf.
      *
-     * @param int $pValue
-     *
      * @return $this
      */
-    public function setXfIndex($pValue)
+    public function setXfIndex(int $pValue)
     {
         $this->xfIndex = $pValue;
 
diff --git a/src/PhpSpreadsheet/Worksheet/Iterator.php b/src/PhpSpreadsheet/Worksheet/Iterator.php
index 6cfed37a..93b0bb04 100644
--- a/src/PhpSpreadsheet/Worksheet/Iterator.php
+++ b/src/PhpSpreadsheet/Worksheet/Iterator.php
@@ -47,20 +47,16 @@ class Iterator implements \Iterator
 
     /**
      * Current Worksheet.
-     *
-     * @return Worksheet
      */
-    public function current()
+    public function current(): Worksheet
     {
         return $this->subject->getSheet($this->position);
     }
 
     /**
      * Current key.
-     *
-     * @return int
      */
-    public function key()
+    public function key(): int
     {
         return $this->position;
     }
diff --git a/src/PhpSpreadsheet/Worksheet/Row.php b/src/PhpSpreadsheet/Worksheet/Row.php
index 4f48a346..cf935743 100644
--- a/src/PhpSpreadsheet/Worksheet/Row.php
+++ b/src/PhpSpreadsheet/Worksheet/Row.php
@@ -41,10 +41,8 @@ class Row
 
     /**
      * Get row index.
-     *
-     * @return int
      */
-    public function getRowIndex()
+    public function getRowIndex(): int
     {
         return $this->rowIndex;
     }
@@ -64,10 +62,8 @@ class Row
 
     /**
      * Returns bound worksheet.
-     *
-     * @return Worksheet
      */
-    public function getWorksheet()
+    public function getWorksheet(): Worksheet
     {
         return $this->worksheet;
     }
diff --git a/src/PhpSpreadsheet/Worksheet/RowCellIterator.php b/src/PhpSpreadsheet/Worksheet/RowCellIterator.php
index 9b9d54eb..6a96a826 100644
--- a/src/PhpSpreadsheet/Worksheet/RowCellIterator.php
+++ b/src/PhpSpreadsheet/Worksheet/RowCellIterator.php
@@ -2,6 +2,7 @@
 
 namespace PhpOffice\PhpSpreadsheet\Worksheet;
 
+use PhpOffice\PhpSpreadsheet\Cell\Cell;
 use PhpOffice\PhpSpreadsheet\Cell\Coordinate;
 use PhpOffice\PhpSpreadsheet\Exception as PhpSpreadsheetException;
 
@@ -59,7 +60,7 @@ class RowCellIterator extends CellIterator
      *
      * @return $this
      */
-    public function resetStart($startColumn = 'A')
+    public function resetStart(string $startColumn = 'A')
     {
         $this->startColumnIndex = Coordinate::columnIndexFromString($startColumn);
         $this->adjustForExistingOnlyRange();
@@ -77,7 +78,7 @@ class RowCellIterator extends CellIterator
      */
     public function resetEnd($endColumn = null)
     {
-        $endColumn = $endColumn ? $endColumn : $this->worksheet->getHighestColumn();
+        $endColumn = $endColumn ?: $this->worksheet->getHighestColumn();
         $this->endColumnIndex = Coordinate::columnIndexFromString($endColumn);
         $this->adjustForExistingOnlyRange();
 
@@ -91,7 +92,7 @@ class RowCellIterator extends CellIterator
      *
      * @return $this
      */
-    public function seek($column = 'A')
+    public function seek(string $column = 'A')
     {
         $columnx = $column;
         $column = Coordinate::columnIndexFromString($column);
@@ -116,20 +117,16 @@ class RowCellIterator extends CellIterator
 
     /**
      * Return the current cell in this worksheet row.
-     *
-     * @return \PhpOffice\PhpSpreadsheet\Cell\Cell
      */
-    public function current()
+    public function current(): ?Cell
     {
         return $this->worksheet->getCellByColumnAndRow($this->currentColumnIndex, $this->rowIndex);
     }
 
     /**
      * Return the current iterator key.
-     *
-     * @return string
      */
-    public function key()
+    public function key(): string
     {
         return Coordinate::stringFromColumnIndex($this->currentColumnIndex);
     }
@@ -166,10 +163,8 @@ class RowCellIterator extends CellIterator
 
     /**
      * Return the current iterator position.
-     *
-     * @return int
      */
-    public function getCurrentColumnIndex()
+    public function getCurrentColumnIndex(): int
     {
         return $this->currentColumnIndex;
     }
diff --git a/src/PhpSpreadsheet/Worksheet/RowDimension.php b/src/PhpSpreadsheet/Worksheet/RowDimension.php
index c4a87bdb..d86dd80f 100644
--- a/src/PhpSpreadsheet/Worksheet/RowDimension.php
+++ b/src/PhpSpreadsheet/Worksheet/RowDimension.php
@@ -43,10 +43,8 @@ class RowDimension extends Dimension
 
     /**
      * Get Row Index.
-     *
-     * @return int
      */
-    public function getRowIndex()
+    public function getRowIndex(): int
     {
         return $this->rowIndex;
     }
@@ -54,13 +52,11 @@ class RowDimension extends Dimension
     /**
      * Set Row Index.
      *
-     * @param int $pValue
-     *
      * @return $this
      */
-    public function setRowIndex($pValue)
+    public function setRowIndex(int $index)
     {
-        $this->rowIndex = $pValue;
+        $this->rowIndex = $index;
 
         return $this;
     }
@@ -78,23 +74,21 @@ class RowDimension extends Dimension
     /**
      * Set Row Height.
      *
-     * @param float $pValue
+     * @param float $height
      *
      * @return $this
      */
-    public function setRowHeight($pValue)
+    public function setRowHeight($height)
     {
-        $this->height = $pValue;
+        $this->height = $height;
 
         return $this;
     }
 
     /**
      * Get ZeroHeight.
-     *
-     * @return bool
      */
-    public function getZeroHeight()
+    public function getZeroHeight(): bool
     {
         return $this->zeroHeight;
     }
@@ -102,11 +96,9 @@ class RowDimension extends Dimension
     /**
      * Set ZeroHeight.
      *
-     * @param bool $pValue
-     *
      * @return $this
      */
-    public function setZeroHeight($pValue)
+    public function setZeroHeight(bool $pValue)
     {
         $this->zeroHeight = $pValue;
 
diff --git a/src/PhpSpreadsheet/Worksheet/RowIterator.php b/src/PhpSpreadsheet/Worksheet/RowIterator.php
index 42542533..469c284c 100644
--- a/src/PhpSpreadsheet/Worksheet/RowIterator.php
+++ b/src/PhpSpreadsheet/Worksheet/RowIterator.php
@@ -65,10 +65,12 @@ class RowIterator implements Iterator
      *
      * @return $this
      */
-    public function resetStart($startRow = 1)
+    public function resetStart(int $startRow = 1)
     {
         if ($startRow > $this->subject->getHighestRow()) {
-            throw new PhpSpreadsheetException("Start row ({$startRow}) is beyond highest row ({$this->subject->getHighestRow()})");
+            throw new PhpSpreadsheetException(
+                "Start row ({$startRow}) is beyond highest row ({$this->subject->getHighestRow()})"
+            );
         }
 
         $this->startRow = $startRow;
@@ -89,7 +91,7 @@ class RowIterator implements Iterator
      */
     public function resetEnd($endRow = null)
     {
-        $this->endRow = ($endRow) ? $endRow : $this->subject->getHighestRow();
+        $this->endRow = $endRow ?: $this->subject->getHighestRow();
 
         return $this;
     }
@@ -101,7 +103,7 @@ class RowIterator implements Iterator
      *
      * @return $this
      */
-    public function seek($row = 1)
+    public function seek(int $row = 1)
     {
         if (($row < $this->startRow) || ($row > $this->endRow)) {
             throw new PhpSpreadsheetException("Row $row is out of range ({$this->startRow} - {$this->endRow})");
@@ -131,10 +133,8 @@ class RowIterator implements Iterator
 
     /**
      * Return the current iterator key.
-     *
-     * @return int
      */
-    public function key()
+    public function key(): int
     {
         return $this->position;
     }
@@ -157,10 +157,8 @@ class RowIterator implements Iterator
 
     /**
      * Indicate if more rows exist in the worksheet range of rows that we're iterating.
-     *
-     * @return bool
      */
-    public function valid()
+    public function valid(): bool
     {
         return $this->position <= $this->endRow && $this->position >= $this->startRow;
     }
diff --git a/src/PhpSpreadsheet/Writer/Xls/Worksheet.php b/src/PhpSpreadsheet/Writer/Xls/Worksheet.php
index a793128a..d2784d6d 100644
--- a/src/PhpSpreadsheet/Writer/Xls/Worksheet.php
+++ b/src/PhpSpreadsheet/Writer/Xls/Worksheet.php
@@ -1715,9 +1715,7 @@ class Worksheet extends BIFFwriter
         $length = 0x0022; // Number of bytes to follow
 
         $iPaperSize = $this->phpSheet->getPageSetup()->getPaperSize(); // Paper size
-
-        $iScale = $this->phpSheet->getPageSetup()->getScale() ?
-            $this->phpSheet->getPageSetup()->getScale() : 100; // Print scaling factor
+        $iScale = $this->phpSheet->getPageSetup()->getScale() ?: 100; // Print scaling factor
 
         $iPageStart = 0x01; // Starting page number
         $iFitWidth = (int) $this->phpSheet->getPageSetup()->getFitToWidth(); // Fit to number of pages wide
diff --git a/tests/PhpSpreadsheetTests/Calculation/Functions/MathTrig/SubTotalTest.php b/tests/PhpSpreadsheetTests/Calculation/Functions/MathTrig/SubTotalTest.php
index 14865673..efee60bd 100644
--- a/tests/PhpSpreadsheetTests/Calculation/Functions/MathTrig/SubTotalTest.php
+++ b/tests/PhpSpreadsheetTests/Calculation/Functions/MathTrig/SubTotalTest.php
@@ -57,11 +57,10 @@ class SubTotalTest extends TestCase
         return require 'tests/data/Calculation/MathTrig/SUBTOTAL.php';
     }
 
-    protected function rowVisibility()
+    protected function rowVisibility($data)
     {
-        $data = [1 => false, 2 => true, 3 => false, 4 => true, 5 => false, 6 => false, 7 => false, 8 => true, 9 => false, 10 => true, 11 => true];
-        foreach ($data as $k => $v) {
-            yield $k => $v;
+        foreach ($data as $row => $visibility) {
+            yield $row => $visibility;
         }
     }
 
@@ -69,10 +68,11 @@ class SubTotalTest extends TestCase
      * @dataProvider providerHiddenSUBTOTAL
      *
      * @param mixed $expectedResult
+     * @param mixed $hiddenRows
      */
-    public function testHiddenSUBTOTAL($expectedResult, ...$args): void
+    public function testHiddenSUBTOTAL($expectedResult, $hiddenRows, ...$args): void
     {
-        $visibilityGenerator = $this->rowVisibility();
+        $visibilityGenerator = $this->rowVisibility($hiddenRows);
 
         $rowDimension = $this->getMockBuilder(RowDimension::class)
             ->setMethods(['getVisible'])
diff --git a/tests/PhpSpreadsheetTests/Worksheet/ColumnCellIterator2Test.php b/tests/PhpSpreadsheetTests/Worksheet/ColumnCellIterator2Test.php
index c542d89e..90280bd2 100644
--- a/tests/PhpSpreadsheetTests/Worksheet/ColumnCellIterator2Test.php
+++ b/tests/PhpSpreadsheetTests/Worksheet/ColumnCellIterator2Test.php
@@ -25,9 +25,11 @@ class ColumnCellIterator2Test extends TestCase
         $lastCoordinate = '';
         $firstCoordinate = '';
         foreach ($iterator as $cell) {
-            $lastCoordinate = $cell->getCoordinate();
-            if (!$firstCoordinate) {
-                $firstCoordinate = $lastCoordinate;
+            if ($cell !== null) {
+                $lastCoordinate = $cell->getCoordinate();
+                if (!$firstCoordinate) {
+                    $firstCoordinate = $lastCoordinate;
+                }
             }
         }
         self::assertEquals($expectedResultFirst, $firstCoordinate);
diff --git a/tests/PhpSpreadsheetTests/Worksheet/ColumnIteratorTest.php b/tests/PhpSpreadsheetTests/Worksheet/ColumnIteratorTest.php
index de985cee..71a05d48 100644
--- a/tests/PhpSpreadsheetTests/Worksheet/ColumnIteratorTest.php
+++ b/tests/PhpSpreadsheetTests/Worksheet/ColumnIteratorTest.php
@@ -67,6 +67,22 @@ class ColumnIteratorTest extends TestCase
         }
     }
 
+    public function testIteratorResetStart(): void
+    {
+        $iterator = new ColumnIterator($this->mockWorksheet, 'B', 'D');
+        $iterator->resetStart('E');
+
+        $key = $iterator->key();
+        self::assertSame('E', $key);
+
+        $lastColumn = $iterator->key();
+        while ($iterator->valid() !== false) {
+            $iterator->next();
+            $lastColumn = $iterator->key();
+        }
+        self::assertSame('F', $lastColumn);
+    }
+
     public function testSeekOutOfRange(): void
     {
         $this->expectException(\PhpOffice\PhpSpreadsheet\Exception::class);
@@ -81,4 +97,12 @@ class ColumnIteratorTest extends TestCase
         $iterator->prev();
         self::assertFalse($iterator->valid());
     }
+
+    public function testResetStartOutOfRange(): void
+    {
+        $this->expectException(\PhpOffice\PhpSpreadsheet\Exception::class);
+
+        $iterator = new ColumnIterator($this->mockWorksheet, 'B', 'D');
+        $iterator->resetStart('H');
+    }
 }
diff --git a/tests/PhpSpreadsheetTests/Worksheet/RowCellIterator2Test.php b/tests/PhpSpreadsheetTests/Worksheet/RowCellIterator2Test.php
index 20d10da9..52183168 100644
--- a/tests/PhpSpreadsheetTests/Worksheet/RowCellIterator2Test.php
+++ b/tests/PhpSpreadsheetTests/Worksheet/RowCellIterator2Test.php
@@ -25,9 +25,11 @@ class RowCellIterator2Test extends TestCase
         $lastCoordinate = '';
         $firstCoordinate = '';
         foreach ($iterator as $cell) {
-            $lastCoordinate = $cell->getCoordinate();
-            if (!$firstCoordinate) {
-                $firstCoordinate = $lastCoordinate;
+            if ($cell !== null) {
+                $lastCoordinate = $cell->getCoordinate();
+                if (!$firstCoordinate) {
+                    $firstCoordinate = $lastCoordinate;
+                }
             }
         }
         self::assertEquals($expectedResultFirst, $firstCoordinate);
diff --git a/tests/PhpSpreadsheetTests/Worksheet/RowIteratorTest.php b/tests/PhpSpreadsheetTests/Worksheet/RowIteratorTest.php
index 919da9ef..c527f434 100644
--- a/tests/PhpSpreadsheetTests/Worksheet/RowIteratorTest.php
+++ b/tests/PhpSpreadsheetTests/Worksheet/RowIteratorTest.php
@@ -65,6 +65,22 @@ class RowIteratorTest extends TestCase
         }
     }
 
+    public function testIteratorResetStart(): void
+    {
+        $iterator = new RowIterator($this->mockWorksheet, 2, 4);
+        $iterator->resetStart(5);
+
+        $key = $iterator->key();
+        self::assertSame(5, $key);
+
+        $lastRow = $iterator->key();
+        while ($iterator->valid() !== false) {
+            $iterator->next();
+            $lastRow = $iterator->key();
+        }
+        self::assertSame(6, $lastRow);
+    }
+
     public function testSeekOutOfRange(): void
     {
         $this->expectException(\PhpOffice\PhpSpreadsheet\Exception::class);
@@ -79,4 +95,12 @@ class RowIteratorTest extends TestCase
         $iterator->prev();
         self::assertFalse($iterator->valid());
     }
+
+    public function testResetStartOutOfRange(): void
+    {
+        $this->expectException(\PhpOffice\PhpSpreadsheet\Exception::class);
+
+        $iterator = new RowIterator($this->mockWorksheet, 2, 4);
+        $iterator->resetStart(10);
+    }
 }
diff --git a/tests/data/Calculation/MathTrig/SUBTOTALHIDDEN.php b/tests/data/Calculation/MathTrig/SUBTOTALHIDDEN.php
index 001531f8..accaf03e 100644
--- a/tests/data/Calculation/MathTrig/SUBTOTALHIDDEN.php
+++ b/tests/data/Calculation/MathTrig/SUBTOTALHIDDEN.php
@@ -15,59 +15,85 @@ $baseTestData = [
     12 => ['A' => 89],
 ];
 
+$hiddenRows = [
+    1 => false,
+    2 => true,
+    3 => false,
+    4 => true,
+    5 => false,
+    6 => false,
+    7 => false,
+    8 => true,
+    9 => false,
+    10 => true,
+    11 => true,
+    12 => false,
+];
+
 return [
     [
         21,
+        $hiddenRows,
         101,
         $baseTestData,
     ],
     [
         5,
+        $hiddenRows,
         102,
         $baseTestData,
     ],
     [
         5,
+        $hiddenRows,
         103,
         $baseTestData,
     ],
     [
         55,
+        $hiddenRows,
         104,
         $baseTestData,
     ],
     [
         1,
+        $hiddenRows,
         105,
         $baseTestData,
     ],
     [
         48620,
+        $hiddenRows,
         106,
         $baseTestData,
     ],
     [
         23.1840462387393,
+        $hiddenRows,
         107,
         $baseTestData,
     ],
     [
         20.7364413533277,
+        $hiddenRows,
         108,
         $baseTestData,
     ],
     [
         105,
+        $hiddenRows,
         109,
         $baseTestData,
     ],
     [
         537.5,
+        $hiddenRows,
         110,
         $baseTestData,
     ],
     [
         430,
+        $hiddenRows,
         111,
         $baseTestData,
     ],