PHP 8.1 compatibility for 2.13

[derrabus]

Q	A
Type	bug
BC Break	no
Fixed issues	N/A

Summary

The methods getDateTimeTzFormatString() and getDateTimeFormatString() will never return null in real implementations, but this is what the mock created in that test currently does.

Their return value is passed to DateTime::format(). Passing null to that method triggers a deprecation warning in PHP 8.1. With this PR, I'd like to fix the broken mock.

[morozov]

Note, DBAL 2.x is only supported in the part of improving the upgrade path to 3.x. I do not think we should be beating the dead horse and adding the support for PHP 8.1 thus increasing the supported version range from PHP 7.1 to 8.1.

[derrabus]

See my comment #4734 (comment)

Let's continue the discussion there.

[derrabus]

@morozov I have backported several changes from #4682 as you suggested, and enabled PHP 8.1 in CI.

[morozov]

Since it comes from a newer major release, I'd double check if it can cause any BC break. Do such changes belong to a patch release at all or the support for a new backwards-incompatible PHP version deserves a minor release?

[derrabus]

All code that I could find that accesses this property already assumes that it is a string. In most cases, the value is used in a string concatenation operation which would implicitly cast it to an empty string if it were null.

[morozov]

Since it comes from a newer major release, I'd double check if it can cause any BC break.

[morozov]

Thanks, @derrabus!