How can I run cron.php via crontab?
You could run the cron URL that Drupal provides in Administration > Configuration > System > Cron. 

And in your crontab, instead of invoking the cron.php file individually (won't work), you just have to invoke a URL. This can be done with wget, lynx, curl, etc. E.g.:

With wget:                             

0 * * * * wget -O - -q -t 1 http://www.example.com/cron.php?cron_key=&lt;key&gt;


Configuring cron jobs using the cron command