Phpstan hasn't identified any errors in Samples in a long time. But, as it turns out, one particular error is suppressed:
```
Variable $helper might not be defined.
```
This error would be generated by almost all samples, and the errors that it suppresses will become problematic if we move to Level 10. We are not yet committed to doing that, but it is pretty easy to write a script to change the samples so that error no longer happens, and there isn't really any reason to delay doing so. The results of that script constitute this PR.
Issue #4435 was initially described incorrectly. While investigating the original description, I came upon this problem. Ods Reader is trying to set some cells to null without supplying a valid DataType to setValueExplicit, causing that method to throw an exception. Reader is changed to no longer call that method when value is null and DataType is null-string.
Document some cases where solution might not work well. Making class extendable and offering some over-rideable methods may allow for solutions to some of these problems.
Fix#4416. A peculiar problem indeed. PhpSpreadsheet has been considering a column to be filtered if any cell in the column is filtered and does not preserve the column width if that is the case. It should consider the column not filtered if any cell in the column is not filtered, and consider it filtered only if there are no cells to which that applies. At least, that's how I think it should work, and this change doesn't break any existing tests, and solves this issue.
It flagged a statement as dead code. It was correct - there was a typo in the variable name. But no tests had failed. The explanation was, of course, that this particular path was not adequately tested.
PhpSpreadsheet extends Excel (dating back to PHPExcel) by allowing the ordinal form of days in the DATE function, implemented as "take the numeric portion if the field is a string consisting of a numeric portion followed by some alphabetics". Whether or not this is a good idea, it would be a breaking change to eliminate it, so that's not going to happen. However, the same logic has been applied to month, and I don't see a use case for that, so I'm eliminating it - any non-numeric string used as the month parameter will now result in a VALUE error. It also turns out that Excel accepts null, false, and true for the month, and PhpSpreadsheet will now do likewise.
Spreadsheet clone already copies Calculation instanceArrayReturnType property. It should also copy some other Calculation instance properties, namely suppressFormulaErrors, calculationCacheEnabled, and branchPruningEnabled.