The project includes an article on "PHP performance tips", which is bordering on being ridiculous. There are of course lots of blog posts with similar nonsense around the web, but since this is being endorsed by Google, it's fair to assume that people might take it at face value. That's unfortunate, as almost none of it is true. The PHP Team responded back with this message, wherein they refute the unfounded nonsense.
He notes that these practices aren't the best road to optimization and, depending on when they might be used, could be harmful to the development of the application. He also recommends XDebug if you really want to get to know where your application is struggling.