First check the system logs, for example /var/log/cron or /var/log/messages and check for entries like
Jan 11 07:30:01 linux crond: (username) CMD (/home/sites/htdocs/chetan/backuptest/create_zip.php >> /dev/null 2>&1)
if the cron jobs is executed and something has gone wrong, strip out the redirection of standard output and standard error from the cron entry and wait for it run again. Then check the mail of the crontab owner (using the mail
command from a terminal) since it is the place where the cron daemon sends stdout and stderr by default (if not redirected elsewhere).
Regarding this specific issue, I found an interesting article here
. Hope this helps.