First use the is_uploaded_file function to check if the file has been successfully uploaded by HTTP POST.
Then use the copy function to save the uploaded file on the server.
copy($_FILE["userfile"]["tmp_name"], "new_filename");
You can now read the file from the server's hard drive to attach it to the email and you are done.
Don't forget that for the file to be successfully uploaded the form enctype must be set to "multipart/form-data"
Hope it helps.