Right along with some of the other posts about the "best practices" that Google recently post, the PHP group has also responded to dispel some of the recommendations Google gives.
With regards to the new article posted at http://code.google.com/ speed/articles/optimizing-php.html, all of the advice in it is completely incorrect. We at the PHP team would like to offer some thoughts aimed at debunking these claims, which the author has clearly not verified.
The group through each of the five things Google recommends and tells why they are incorrect including the fact that, while some of the recommendations might have been true back in PHP3, they aren't at all true of the current PHP5 releases.
Takes these with a grain of salt and incorporate where applicable.
Most importantly, study the code, hack it (in the old school sense), and gain a deep understanding of it.