Some light reading: How to compile PHP from Source. See also: Where’s the php-src/PHP-Internals Main Entry Point?.
Also good:
Tag Archives: type system
Everything you need (and don’t need) to know about PHP’s type system
I read Everything you need (and don’t need) to know about PHP’s type system, it was interesting. My favourite quote was at the end:
“I believe php’s type system is incredibly rich and carries many innovative and legacy features and they all make much sense when you look at the history of the language development.”