Updates to documentation concerning Value Binders

Fix a couple of checks on characters at positions within a string before checking the length of the string when the offset may not exist

docs/topics/accessing-cells.md

@@ -110,6 +110,11 @@ values beginning with `=` will be converted to a formula. Strings that
 aren't numeric, or that don't begin with a leading `=` will be treated
 as genuine string values.
 
+Note that a numeric string that begins with a leading zero (that isn't
+immediately followed by a decimal separator) will not be converted to a
+numeric, so values like phone numbers (e.g. `01615991375``will remain as
+strings).
+
 This "conversion" is handled by a cell "value binder", and you can write
 custom "value binders" to change the behaviour of these "conversions".
 The standard PhpSpreadsheet package also provides an "advanced value
@@ -138,8 +143,10 @@ Formats handled by the advanced value binder include:
 - When strings contain a newline character (`\n`), then the cell styling is
   set to wrap.
 
-You can read more about value binders later in this section of the
-documentation.
+Basically, it attempts to mimic the behaviour of the MS Excel GUI.
+
+You can read more about value binders [later in this section of the
+documentation](#using-value-binders-to-facilitate-data-entry).
 
 ### Setting a formula in a Cell
 
@@ -551,8 +558,13 @@ $spreadsheet->getActiveSheet()->setCellValue('A5', 'Date/time value:');
 $spreadsheet->getActiveSheet()->setCellValue('B5', '21 December 1983');
 ```
 
-**Creating your own value binder is easy.** When advanced value binding
-is required, you can implement the
-`\PhpOffice\PhpSpreadsheet\Cell\IValueBinder` interface or extend the
+Alternatively, a `\PhpOffice\PhpSpreadsheet\Cell\StringValueBinder` class is available
+if you want to preserve all string content as strings. This might be appropriate if you
+were loading a file containing values that could be interpreted as numbers (e.g. numbers
+with leading zeroes such as phone numbers), but that should be retained as strings.
+
+**Creating your own value binder is relatively straightforward.** When more specialised
+value binding is required, you can implement the
+`\PhpOffice\PhpSpreadsheet\Cell\IValueBinder` interface or extend the existing
 `\PhpOffice\PhpSpreadsheet\Cell\DefaultValueBinder` or
 `\PhpOffice\PhpSpreadsheet\Cell\AdvancedValueBinder` classes.

src/PhpSpreadsheet/Cell/DefaultValueBinder.php

@@ -57,11 +57,11 @@ class DefaultValueBinder implements IValueBinder
             return DataType::TYPE_STRING;
         } elseif ($value instanceof RichText) {
             return DataType::TYPE_INLINE;
-        } elseif (is_string($value) && $value[0] === '=' && strlen($value) > 1) {
+        } elseif (is_string($value) && strlen($value) > 1 && $value[0] === '=') {
             return DataType::TYPE_FORMULA;
         } elseif (preg_match('/^[\+\-]?(\d+\\.?\d*|\d*\\.?\d+)([Ee][\-\+]?[0-2]?\d{1,3})?$/', $value)) {
             $tValue = ltrim($value, '+-');
-            if (is_string($value) && $tValue[0] === '0' && strlen($tValue) > 1 && $tValue[1] !== '.') {
+            if (is_string($value) && strlen($tValue) > 1 && $tValue[0] === '0' && $tValue[1] !== '.') {
                 return DataType::TYPE_STRING;
             } elseif ((strpos($value, '.') === false) && ($value > PHP_INT_MAX)) {
                 return DataType::TYPE_STRING;