HOWTO? Adding functions to PHP3

I am trying to add my own internal functions to PHP3, and I seem to be missing some documentation. I have printed out the PHP3 manual, and all I can find in it is some documentation on how the function should be constructed, parameter passing, memory management, etc. There does not seem to be anything that I can find that actually states what other files need to be updated so that the function is linked into PHP3's internal command tables so that the parser, etc can find it or even the accepted way of updating makefiles (hack configure.in ?) so that my code can be optionally selected at compile time and built in. I have had a search through the mail list archives, but none of my queries seem to have produced any worthwhile results. Can anyone refer me to a useful HOWTO on how to add new internal functions to PHP3 ? Thanks, Sean