Handle false return in substr with PHP < 8

[herndlm]

Closes phpstan/phpstan#6939

https://www.php.net/manual/en/function.substr.php#refsect1-function.substr-changelog

Initially I duplicated the benevolent union from the stubs and added a new method to PhpVersion, buuuut let's see if #1061 takes care of that :)

Related: 9ee914b

[herndlm]

I love how easy #1061 made the fix here by falling back to the correct stubs :)

[herndlm]

Hmm, the failing integration tests look like valid pre PHP 8 errors to me (see https://3v4l.org/eu588), but not sure how to "handle" them. Target 1.6.x maybe? :D

I'm going to open PRs over there. UPDATE: nope, not that easy, phpstan fails in slevomat/coding-standard then with

 ------ ----------------------------------------------------
  Line   Helpers/AnnotationTypeHelper.php
 ------ ----------------------------------------------------
  343    Casting to string something that's already string.
 ------ ----------------------------------------------------

same thing for nextras/orm

advise please :) I think similar things regularly happend in the past and were fixed with/after a release, but I don't want to be the one that breaks PHPStan CI :/ I'll happily open PRs though in those projects fixing it and upgrading phpstan if you want to release it like that

[ondrejmirtes]

It's fine, you can fix those projects after the release but I'll just regenerate their baselines so that our CI is green :)

The situation in slevomat CS is a bit complicated because PHP 7.x is still supported by PHPStan is ran only on PHP 8.

[ondrejmirtes]

Thank you.