including php from third party site in header

[JonnyQuest]

I need to include some php in the header of my board. For example, one of our partners uses php and has a search function that they include on their site. We want to include this in our header.
If I parse my headers for php can I simply use the include() function to do this? Do I need to require and files or have an include path? I've never parsed my headers for php so I'm not sure what this entails.
Thanks!