Rasmus Lerdorf, creator of PHP, gave an interesting talk at DrupalCon 2008 in Hungary last month. It mostly concerned PHP performance: how to profile CPU usage and disk access, graph object hierarchies and discover which parts of a framework are unnecessary or poorly optimised.
There were mentions of include_once and the need to optimise the include path to minimise misses, and a strong message that it's best to use PHP purely for the presentation layer and database queries and move the "business logic" into efficient C modules.