With Cron you can run php scripts every minute but not in shorter period of time. So what can you do when you need to run your script every 15 seconds or even 1 second ?

Create shell sctipt like this one fasterthancron.sh

#!/bin/bash
#This script run every seconds
while (sleep 1 && php /path_to_script/faster_than_cron.php) &
do
  wait $!
done

Reference :
Ivan Đurđevac : Run scripts on Linux/Ubuntu every few seconds, faster than Cron can do it..