Found this pdf of George Schlossnagle, the PHP Guru author of Advanced PHP Programming with the set of slides he used for a tutorial at the Zend Conference about Scalable Applications. Its from 2005 but i found it very interesting to read, worth having a look at it.

You can grab it here:

http://www.schlossnagle.org/~george/talks/ZendPerf.pdf

Cheers

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