So much of the web is powered by PHP. Wikipedia (MediaWiki software) and Facebook are two of the largest footprints of usage. And of course, WordPress. PHP has a lot of firepower behind it.
The Facebook www code is written in Hack, which started off as a PHP-compatible language / stdlib, and then dropped PHP compatibility when it was holding them back.
Having worked with both, any one of the major additions (XHP, good type annotations, sane collection types, etc) makes my development experience 10x better; going back to vanilla PHP now just makes me sad...
