Repeating tasks by time,Can anyone help?

Hello , I have a question , How can we repeat a job by time in php? I mean does something like a timer exist in php, which can repeat a task for example in every 5 minutes? And how can a program become resident on server, I mean I have to write a php file, which should be always active,it should have some timers(or with any other ways) so that in certain intervals it should make a query to the database. Can anybody help How I can implement such a thing? Thank you so much, N.Dezhkam. __________________________________________________ Do You Yahoo!? Yahoo! Finance - Get real-time stock quotes http://finance.yahoo.com