create an ASCII text file called "crontab" and add:
0,5,10,15,20,25,30,35,40,45,50,55 * * * * /path/to/php /path/to/script.php >/path/to/log 2>/path/to/error
replace /path/to/php with the location of your php executable.
replace /path/to/script.php with the location of your script.
replace /path/to/log with the location of where you want to script's output to go.
replace /path/to/error with the location of where you want to script's errors to go.
the exact location of where the crontab file needs to go varies with different OS's but usually in /etc
http://www.ussg.iu.edu/usail/automation/cron.html