A cronjob is program which is enable UNIX users to execute scripts(group of command at specific time period.Its normally used sys admin commands. Cronjob is a very useful tool when you want to run some scripts automatically.
To setup a cronjob follow the steps.
Log into your cPanel
Click Cron jobs icon under advanced section and follow the below steps.
Enter an email address ( the cron output will be sent to this email )
Setup the time when the script will be execute.
Enter the full path to your script to be executed like
Note:Please Change [username] with your username and the path of your php script (send_mail.php) in above syntax.
Finally Click on the ‘Add New Cron Job’ button