[php-src] Issue #20589: preg_replace_callback $count number of matches, number of replacements

Issue: https://github.com/php/php-src/issues/20589
Author: kkmuffme

### Description

The following code:

https://3v4l.org/0E8mn#v8.5.0

```php
<?php

$before = 'abca';
$after = preg_replace( '/a/', 'a', $before, -1, $count );
if ( $after === $before ) {
    echo "NO REPLACEMENTS" . PHP_EOL;
}

echo $count . " REPLACEMENTS";
```

Resulted in this output:
```
NO REPLACEMENTS
2 REPLACEMENTS
```

But I expected this output instead:
```
NO REPLACEMENTS
0 REPLACEMENTS
```

https://www.php.net/manual/en/function.preg-replace.php
>If specified, this variable will be filled with the number of replacements done.

Technically, this is correct, since the replacement is identical to the search string.
While for preg_replace this makes practically no difference (except for cases where backslash is incorrectly escaped, you could end up with a preg that says it replaced even though it didn't change the input)

Practically for `preg_replace_callback` this means however, that you always have to compare before/after to be sure whether anything was replaced at all.
And there's no way of knowing how many replacements were actually done.

Changing the behavior should generally be safe I guess, but otherwise just adding an additional param "$replacement_count" for preg_replace_callback would make sense? (so the 1st param docs would be changed to be "number of matches")



### PHP Version

```plain
8.5
```

### Operating System

_No response_