Everyone is just going to be a fanboy here, of course. So let me point out that a few years ago Matt Mullenweg said "none of the most requested features for WordPress would be any easier (or harder) if they were written for PHP 4 or 5 or Python," and advocated that we "continue to maintain PHP 4 as like a PHP-lite. Make it harder, better, faster, stronger." He also said: "PHP 5 has been, from an adoption point of view, a complete flop. Most estimates place it in the single-digit percentages or at best the low teens, mostly gassed by marginal frameworks. Even hosted PHP-powered services who have no shared host compatibility concerns like 30boxes, Digg, Flickr, and WordPress.com, have been slow to move and when they do it will probably be because of speed or security, not features."