Add checks for each mb_* function separately

[capuderg]

As discussed in this PR: symfony/polyfill-mbstring#6, this would be a potential fix for any corner cases.

Currently, we have issue with mb_strtolower not being polyfilled, because the current check only checks if mb_strlen exists.

In WordPress the mb_strlen function is polyfilled before this package can be required, but the mb_strtolower is not (don't know why WP is polyfilling just a few).

With this change, all mb_* functions can be polyfilled separately.

[stof]

If we decide that we want to go that road, we should do it for all polyfills.

[nicolas-grekas]

If we decide that we want to go that road, we should do it for all polyfills.

I agree - and then, what is now a few tenths of extra checks might become hundreds. We should definitely run some benchmarks before doing this.

[JanJakes]

@nicolas-grekas In many cases (like PHP version polyfills) you won't add much, almost nothing in fact, since most of the IFs are already there (i.e. https://github.com/symfony/polyfill/blob/master/src/Php54/bootstrap.php).

And then some polyfills may not need this at all (classes? not sure which ones).

And I guess even if you add hundreds it will still be a fraction of a millisecond but yes, a benchmark would be good to prove such a guess.

[stof]

@nicolas-grekas isn't function_exists optimized by OPCache, so that it is optimized-away entirely when you have the extension loaded anyway ?

[JanJakes]

@stof Seems you're right: https://github.com/php/php-src/blob/e141592da6cdb15709e4aeaab5421c9a46564849/ext/opcache/Optimizer/pass1_5.c#L349

See also: https://stackoverflow.com/questions/21619939/does-function-exists-cache-its-queries/21620062#21620062

[JanJakes]

I prepared a PR handling this across all polyfills - #252. Let's see where it leads us.