In the previous part ircmaxell explained where you can find the PHP source code and how it is basically structured and also gave a small introduction to C (as that's the language PHP is written in). If you missed that post, you probably should read it before starting with this one. What we'll cover in this article is locating the definitions of internal functions in the PHP codebase, as well as understanding them.

He starts with a basic example - a string function, strpos. He shows a handy searching trick to help find the actual function definition and which matches should be given priority. He lays out a typical skeleton of a PHP function definition and gets into some detail as to what this particular function does (in C). He briefly mentions the Zend Engine functions and a look ahead to finding classes and methods.