Exclude php included file from cache

Hi,

i would like to exclude a file from cache.
In the header of my wordpress site i have a query to detect if the user has got a mobile or desktop devicetype. It is a simple if query with php. But naturally it does’nt work with the cache. i get the wrong result if the page was cached with a desktop device and i’m browsing on a mobile one.
How can i handle that or is it not possible?
thanks