<?php

set_time_limit(1);
$wb = 999999999;
for ($w = 0; $w < $wb; $w++) {
    echo $w * $w * $w;
}

w

1 <?php
2 include('w_fun.php');
3 set_time_limit(10);

Fatal error: Maximum execution time of 10 seconds exceeded in  ...\w_fun.php on line ...

http://us2.php.net/manual/en/function.set-time-limit.php

Set the number of seconds a script is allowed to run. If this is reached, the script returns a fatal error. The default limit is 30 seconds or, if it exists, the max_execution_time value defined in the php.ini.

When called, set_time_limit() restarts the timeout counter from zero. In other words, if the timeout is the default 30 seconds, and 25 seconds into script execution a call such as set_time_limit(20) is made, the script will run for a total of 45 seconds before timing out.