The time limit doesnt work correctly 
when PHP sleeps this time IS included when determining the maximum time that the script has been running  


the manual states:
The set_time_limit() function and the configuration directive max_execution_time only affect the execution time of the script itself. Any time spent on activity that happens outside the execution of the script such as system calls using system(), the sleep() function, database queries, etc. is not included when determining the maximum time that the script has been running. 

if you want to reproduce the problem just run this few lines
and you will see it never never echos "after sleep"


<?php
echo "before sleep";
set_time_limit(5);
sleep(6);
echo "after sleep";
?>