Semi-newbie Question .... streams ...

I'm working on one my first PHP projects, and I have extensive experience with coding, so I know what I want to do is not outrageous. The short version is: I need to master the download stream for a file, while delivering it to the enduser, I need to change some bytes in the file data. A simple search and replace. I want to do it in the stream, don't want to create a new file each time. The data is not a string, it is in fact a serial number being inserted into an "EXE" download. I "get" how to do this with ASP, but the active replace in PHP has me totally lost. Any hints ? -- Paul S. Farr