Replace a builtin function

Hello! Is it possible to replace an existing PHP function ( like mail() ) in an extension. I've been patching PHP to have a custom mail() function, but I think replacing the mail function with an extension would be better and will save a lot of time. Can somebody give me a clue or an extension that replaces a built-in function so I can study it ? Thanks! Sorry if this is not the right place for my question ... Yours, Aron