How to assign cron jobs through php

Is it possible to schedule some tasks in php to run automatically on the server at a开发者_开发问答 certain time of the day? . I heard abt cron jobs. is there any way to set it via php code

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http://php.net/system lets you to run any command-line utility.

As for the certain command to set a cron job you have either to google a little or ask on serverfault.