The execution is automatic, you need to write also the command path. for example:
30 * * * * /usr/bin/php /srv/cron/mail.php
Otherwise add #!/usr/bin/php at the top of your script and make the file executable:
// . . .
In command line use chmod: chmod +x mail.php
And in your crontab you can write:
30 * * * * /srv/cron/mail.php
At specific time the server will just run the script. Then you can go to /var/log to check crontab executions and you can also add error_log() to write a specific log about the excution of the functions in your script: http://php.net/manual/en/function.error-log.php