Question

Is it possible to set a “callback function” that do something instead of returning an error whe the time limit is reached?

Answer 1

You could remove the time limit (set_time_limit(0)), and then keep track of the time the PHP script started executing and make a comparison with the current time.

<?php

$startTime = microtime();

// Whatever you need to do...

// Place this where it needs to be in the context of your application.
if (microtime() - $startTime > 15000) { 
    // Time's up!
}