Fix the Collator return types in phpdoc

[marien-probesys]

The types were incoherent with the PHP documentation. There are still some issues with types (e.g. constructor should not accept null value). But changing them would break the API so I thought it wasn't a good idea to fix them yet.

Reference: https://www.php.net/manual/class.collator.php

Contribute to #459

[marien-probesys]

I didn't change the comment because the PHP documentation itself is incoherent (the return type is int, but the doc mentions a false value), see https://www.php.net/manual/en/collator.getstrength.php

[stof]

could it be that older PHP versions can return false for failures while new versions throw exceptions ?

[marien-probesys]

I think it is! I think the relevant change is php/php-src@1b93cfe#diff-22c8fd8eefcbc1be5e379b048ce5726a61b34dc4352d7d2568e76c56095f2c25 (PHP 8)

[stof]

e.g. constructor should not accept null value

We must accept null due to the way internal arguments for scalars are all accepting null even when not marked as nullable (see the deprecation introduced in PHP 8.1), while userland types were always stricter in that regard.

[nicolas-grekas]

Thank you @marien-probesys.