On the excellent PHP section of Sitepoint Thomas Punt has written a good high ievel overview of how PHP code is executed.

There’s a lot going on under the hood when we execute a piece of PHP code. Broadly speaking, the PHP interpreter goes through four stages when executing code: - Lexing - Parsing - Compilation - Interpretation This article will skim through these stages and show how we can view the output from each stage to really see what is going on. Note that while some of the extensions used should already be a part of your PHP installation (such as tokenizer and OPcache), others will need to be manually installed and enabled (such as php-ast and VLD).