Does your script support running from the CLI? ("php -f /path/to/your/script.php")

Thanks Kamal. My script doesn't support running from the CLI. I'm using codeigniter framework. I want to run like http://mydomain.com/controller/function. Is it possible?
Any idea pls..

http://ellislab.com/codeigniter/user-guide/general/cli.html

Set a cronjob that runs the following command:

php -f /path/to/index.php controller function

Thanks kamal & Mika singh. I found an excellent article.
http://net.tutsplus.com/tutorials/php/managing-cron-jobs-with-php-2/

@farhan.sheron that solution is over engineered for a problem as simple as yours. Just add it in manually, you don't need to code a cronjob manager in php.

Kamal, I'm confusing now. my url is working fine via CLI. Eg: http://mydomain.com/cron/sendEmail. I tested in cmd. Now I want to run this url everyday midnight automatically. I successfully installed ubuntu and nginx only. Now what are the steps I have to do? Thanks in advance.

You can do this to run command every midnight:

crontab -e (This opens the crontab editor)

0 0 * * * your_command_here >/dev/null 2>&1

This will run your specified command at 12 AM midnight according to local server time.