[PHP] File content type

[Matt]

Hi, 

I've written a script to upload image files to a server but need to check to make sure 
there image files so I used the image type posted by the browser but that changes 
depending on the system/browser set-up. What I need to do is open the file once its on 
the server then get the content type so it will always be the same if its valid.

Does anyone know how to do this?

Thanks,
Matt