You mean a job that's in progress? Your script can sleep() as long as it wants to keep the job from progressing further. If you mean editing the crontab file, you can edit the crontab files directly (maybe in /var/spool/cron) but be very careful about permissions. You'll most likely have to use sudo to grant access to copy the new file over, and whatever you do, don't allow a web-accessible PHP script to edit root's crontab.