Include can be used for php files.
You may also want to try require_once, because if you include a file that includes a file that has already been included, you will run into errors.
Including php files with commonly used functions, etc... is good practice and should be used for the sake of modularity. There's nothing wrong with include() for php files, but you should try to avoid nested includes. Also, again, I recommend using require_once(). Although note that with require() or require_once() the script will fail if it cannot find the file.
-Jim Keller
http://www.centerfuse.net