Recently on his blog Jack Diederich took a look at three different interpreters for Ruby, PHP and Python to see how they were implemented as compared to the language they're used in.
The other day I went poking around the Ruby and PHP interpreters (the current stable versions). I hadn't looked inside PHP since the 4.x series and Ruby I had never checked out. Like CPython the internals of both PHP and Ruby look something like their resulting language, but in C. For each interpreter I just compiled it and looked at how core types and extension types were implemented.
For PHP, he went with the PHP 5.2.13 release and talks about the compile process (a bit spammy), running the unit tests that come included and how the interpreter handles data types, core types and objects.