Use PHP to create, edit and delete crontab jobs?

Is it possible to use PHP to create, edit and delete crontab jobs?

I know how to list the current crontab jobs of the Apache user:

$output = shell_exec('crontab -l');
echo $output;

But how to add a cron job with PHP? 'crontab -e' would just open a text editor and you will have to manually edit the entries before saving the file.

And how to delete a cron job with PHP? Again you have to manually do this by 'crontab -e'.

With a job string like this:

$job = '0 */2 * * * /usr/bin/php5 /home/user1/work.php';

How do I add it to the crontab jobs list with PHP?