Question on file upload

Hello, I want to change the way file upload processing is done in php. We have a http server (appweb) that puts the uploaded file to a temp directory. This file has headers at the beginning and end. I am looking at code rfc1867.c. This code takes the temp file and seems to extract the real file. I want to change it so instead of entire file, I can run a system awk command to trim the header lines to get the actual file. Has anyone done something similar? I cant find the place where the actual file is being read? Can someone please point to that. This is php 5.2 on bsd. Thanks