Help with cron

I am running a cron job where i need my php scripts to update my crontab file. But after updating the file i cant execute the crontab command from php. I need to do this otherwise the cron will still be running the old contents of the file. I've tried using system(), shell_exec(), exec() but they don't seem to work. Nothing happens, the old crontab is still running. I know system() works because i've done system(ping www.....com) and it works. Hope someone can help!

Are you specifying the full path to crontab? (/usr/bin/crontab) and the full path to your cron file?