[php-src] Issue #15377: preg_match_callback()

Issue: https://github.com/php/php-src/issues/15377 Author: nevez ### Description I think it would be helpful when extracting sections or generally processing very large strings to have a preg_match_callback() function. Currently I use preg_replace_callback(), discarding the return value, but this approach probably has a non-trivial overhead due to string copying and concatenation. It also feels a bit "hacky". ``` preg_replace_callback($regex, function ($matches) use (&$selected) { $selected .= $matches[1]; return ''; }, $block); ``` An hypothetical preg_match_callback() could return void or even better the number of matches, as with preg_match_all(): ``` $nSelected = preg_match_callback($regex, function ($matches) use (&$selected) { $selected .= $matches[1]; }, $block); ```