I've studied this one a few times and it's very much a positive feedback loop. Rewind to 2005: Django and Rails had just been released. PHP had already been ou

I've studied this one a few times and it's very much a positive feedback loop. Rewind to 2005: Django and Rails had just been released. PHP had already been out for 10 years and there were dozens of large systems - PostNuke, phpBB, WordPress, Drupal - that were already out there being useful. For many hosts, their customers wanted to get a blog, forum, etc online quickly and easily. The vast majority were written in PHP, so they set up with PHP by default. Throw in the fact that most of these companies deployed Cpanel (or similar) with one-click installers like Fantastico, etc - which dealt with PHP exclusively iirc - and it was a no-brainer. The availability of apps drove PHP deployments. The sheer number of PHP deployments drove the installation of the same apps.