Books on PHP guts

Hi folks. I'm not looking to write new PHP extensions per se, but am looking to better grok the guts of PHP itself. (That's a first step on the way to writing new extensions, though. Gateway drug!) I'm especially interested in the memory/performance implications of various techniques. Are there any good books on the subject that would be of help? I'm familiar with Sara Goleman's book[1], which has generally good reviews, but it's several years old now and I'm not sure if there's anything newer that covers PHP developments since the 5.0 days. Any suggestions? --Larry Garfield [1] http://www.barnesandnoble.com/w/extending-and-embedding-php-sara-golemon/1006978211