Continuous XML fetching...

[eluser]Unknown[/eluser]
Hi,
I have just started using code igniter, and I am already getting on very well with it, due to my past experience in a wide array of languages!
There is one thing I don't know how to go about doing, both with code igniter and in PHP.

I simply want to fetch an XML file every xx seconds, which will in my case then input the data from the XML into a database.
I don't know if there is a main loop, or where should I make one. I have no idea how to go about this!
Any suggestions will be appreciated! Thanks