In this recent post to his site Ulrich Kautz takes a look at an interesting development in the PHP framework world - C-based frameworks installable as PHP extensions. He covers some of the good and bad things about this approach.
At the End of 2012 I had my first contact with a C-based PHP frameworks, namely YAF. Coincidently, some day afterwards Bruno from phpmaster.com pointed me towards Phalcon - a more modern interpretation of the same idea. So I was hooked.
In his "good idea" category he notes that it's faster because it's already loaded in on the request (no long list of includes) and the memory footprint is less than a PHP equivalent. The "bad" side of things mentions some pretty major hurdles though, including the small communities vs larger ones on PHP-based frameworks and the issues that could come with debugging/upgrading.