Push everything you can into the compile phase

This means that you should never have conditional function or class definitions. So avoid code like this:

This also means you should avoid implicit conditional operations such as autoload and once again the include_once and require_once mechanisms. So, always do includes, function declarations and class definitions at the top level of your scripts. Keep your include trees simple and straightforward and hopefully relative to your including script, or at the very least include from the first directory of your include_path list.