The $callback parameter of preg_replace_callback() should be a callable which accepts an array of strings and returns a string. Its signature can be updated to reflect this.

From https://www.php.net/manual/en/function.preg-replace-callback.php:

A callback that will be called and passed an array of matched elements in the subject string. The callback should return the replacement string. This is the callback signature:

handler(array $matches): string

The $matches parameter is of type array<int|string, string> because using a named capturing group results in an array element keyed by the capturing group name. Example: https://3v4l.org/bDNbd

Interestingly the callback function can also return an empty value such as null, false, or void and PHP doesn't complain about it, but I would consider it a bug if the callback returns anything other than a string. This might warrant further discussion in case there's a common use case I've not considered.