Browsing around the php.net website (something that I do with a certain frequency for different reasons), I came across LXR, the PHP4 source code cross reference.
Based on a tool originally developed for aiding in the exploration of the Linux kernel source code, LXR makes it possible to navigate the PHP source tree in a completely interactive way--every function call, macro call and user-defined data type is hyperlinked to its definition.

I know a lot of questions that can be answered by this handy tool. You just put in the function/variable that you want to search through the source for, and it spits out some links to the places in the PHP source code that it sees that term. You can also just navigate through the source for PHP4, the Zend Engine (original and 2), and the TSRM source as well.

It's a pretty cool little tool, and much easier than trying to sort through a grep of the source. I just wonder why it hasn't gotten much publicity...

Thanks to the php|architect blog for this link...