Check that we are not using features deprecated in PHP 8

[javiereguiluz]

Description
PHP 8 won't be released anytime soon, but it has already removed a lot of features, functions, settings, etc. that were deprecated in PHP 7.2 and 7.3.

We probably don't use many (any?) of these, but to make our code future-proof we should check this comprehensive list: php/php-src#3770

[stof]

Well, if they haven't messed up with triggering E_USER_DEPRECATED, we should already be fine, as any E_USER_DEPRECATED would be detected by our testsuite (and by the profiler in dev).

[javiereguiluz]

Not sure about that. For example, in our Yaml component code we have code like this: ... & (int) ini_get('mbstring.func_overload') but mbstring.func_overload no longer exists in PHP 8.0.

[stof]

Well, our YAML component does not rely on mbstring.func_overload. It makes sure to keep working when you use that feature.

AFAICT, ini_get does not break for unknown settings (otherwise, the YAML component would already be broken if mbstring is not loaded), and so this code would simply never enter inside the code running only when the mbstring.func_overload feature is used.
Deleting that code will only be possible when PHP 8 is our min version (so that our code cannot run on a runtime having mbstring.func_overload enabled).

[nicolas-grekas]

I'm closing as PHP8 is a remote and moving target. Test suite will tell us where we'll need to update, as done for previous versions.