Fix GH-20589: Add PREG_REPLACE_COUNT_CHANGES flag for counting actual replace

[jordikroon]

This is a fix/implementation for #20589

Add a new flag PREG_REPLACE_COUNT_CHANGES to preg_replace() and preg_replace_callback() that counts only effective changes rather than all matched replacements. When this flag is set, the replacement count is incremented only when the replacement differs from the original matched text, allowing callers to distinguish between identity replacements and actual content changes.

• Add PREG_REPLACE_COUNT_CHANGES constant definition
• Extend php_pcre_replace() and php_pcre_replace_impl() signatures with flags parameter
• Implement change detection logic comparing replacement with original match
• Update function signatures throughout the codebase (php_pcre_replace_array, php_replace_in_subject, _preg_replace_common)
• Add test coverage for identity replacements, backreferences, and callbacks
• Update stub definitions and internal function signatures

[ndossche]

I don't like this because this won't work in general. Simple proof below.

<?php
function callback($arr) {
    return match ($arr[0]) {
        'a' => 'ab',
        'bba' => 'ba',
    };
}

$after = preg_replace_callback('/^a|bba/', 'callback', 'abba', -1, $count, PREG_REPLACE_COUNT_CHANGES);
echo $after,"\n"; // Identical to input
var_dump($count); // int(2)

[jordikroon]

Nice catch @ndossche. Didn't think of that. I have updated the implementation to cover that. Also added some more tests that I could think of.

[ndossche]

I'm not sure this can be fixed in general. See this small change that results in a change count of 2, while you could argue that the result should be 1:

<?php
function callback($arr) {
    return match ($arr[0]) {
        'a' => 'ab',
        'bba' => 'baa',
    };
}

$after = preg_replace_callback('/^a|bba/', 'callback', 'abba', -1, $count, PREG_REPLACE_COUNT_CHANGES);
echo $after,"\n";
var_dump($count);

I think the main problem is that it's going to be hard to make the change count well-defined.