New Looming Problems with PHP8.1

[oleibman]

More deprecations. The following corrections are made in this PR:

• Calculation.php has a call to ctype_upper and apparently one of the samples manages to pass it an int. That function treats int differently from numeric strings, and that treatment is on the deprecation list. Enclosing the argument in quotes cannot cause a problem unless the int represents the ASCII value of an uppercase letter, which I cannot believe is the case; anyhow, if it is, the code will wind up with a nonsense result, e.g. if column is C and row is 1, the cell will be resolved as C1, but if column is int 67 (ASCII for C) and row is 1, the cell will be resolved as 671, not C1.
• Several Worksheet iterators need one or more functions to explicitly declare their return types. Thankfully, this does not seem to break earlier PHP versions.
• LocaleFloatsTest - see issue PHP8 breaks Writer/Xlsx/LocaleFloatsTest; Probably Not Fixable #1663. This was supposed to fail in PHP 8.0, but var_dump continued to support the old way (for 64-bit PHP only, not for 32-bit). PHP 8.1 appears to correct that omission, and the test will now fail. It doesn't show up as a failure in Github because of an accident - the attempt to set the locale to France in Github fails, so it skips the test before attempting the var_dump. But it does fail locally on my system. I have changed the test to use sprintf rather than var_dump; I think users are far more likely to use sprintf rather than var_dump in their applications. (They are, of course, even more likely to just cast to string, but the result of doing that is already different in 8.0 than in 7.4.) I would be equally happy to delete the test altogether.

There remain PHP 8.1 problems with Mpdf which are, of course, out of scope here.

There is one additional problem that I do not address in this ticket. The auto_detect_line_endings setting is being deprecated. This has some implications for Csv. I have another PR ready for Csv, and will discuss that problem there.

This is:

- [x] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?