The code is ugly as sin; but it works... I'll do some refactoring and cleaning later (once I've had some sleep, because I'm stupidly still working on this at 3am)
The main remaining issue is formulae that can't be parsed in BIFF8 files; e.g. formulae that use functions that aren't available. In this case, all CF in that worksheet is corrupted, and the file errors when opened, so it is a serious issue. The ISODD()/ISEVEN example in 07_Expression_Comparisons.php uses these, so I've temporarily commented out setting that CF range until I've solved that problem. There are other limitations listed in the BIFF documentation; but they're harder to detect.
I've also left a couple of debug statements in the code to display these formula errors: I'll remove them once I've resolved the issue.
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