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The PHP Manual has a (sadly mostly empty) chapter on PHP internals.
The main development mailing list is internals@lists.php.net. You can sign up via php.net and/or use Markmail to search the archives.
The git repository for PHP is located on git.php.net, but there is also a mirror on GitHub.
For browsing the source code you should use the lxr.php.net cross reference tool.
The PHP wiki has a list of various resources on PHP development (blog posts, books, slides, etc).
In particular there is an (older) book by Sara Golemon: Extending and Embedding PHP.
A more current and ongoing effort is http://www.phpinternalsbook.com
If you have questions, you should try the #php.pecl room on efnet.
Also see this presentation by Sebastian Bergmann about Compiler Internals:
And make sure to check Nikic's blog. He's got a number of posts on how to read the source:
In addition to that, you can check the PHP Credits for individual c