It’s always nice when a late answer to a Stack Overflow question shows up with more information than you wanted/needed. The specifics here on dealing with the thread safe resource manager are worth it (if you’re looking for that sort of thing), but more important are the links to the two personal blogs covering random adventures in the PHP source code. These sorts of oral/informal technical writeups are invaluable when you’re learning to reason about any long running software project.
More PHP Internals Resourcesastorm
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Quick dispatches from the life of a working programmer.