In this recent post to the SitePoint PHP blog Bruno Skvorc compares two technologies that have influence how PHP performs - the HHVM (HipHop Virtual Machine) and the Zend Engine in PHP6 - and the potential replacement of one with the other.
The fabled PHP 6 is long overdue. This unicorn of the web dev world has been “coming” for decades now, and it’s still not clear whether or not it’s actually something that’s going to happen in this decade, or just an idea, a fantasy of the PHP userbase. [...] In the latest edition [of the PHPClasses podcast] between Manuel Lemos and César Rodas, an interesting topic arose among others – Facebook’s HHVM replacing Zend Engine in PHP 6. While this was purely speculation on the part of the participants, and whether or not you believe in PHP 6, you have to admit it’s an interesting notion.
He starts with a look at the overall pros of the HHVM solution - the speed of execution, that it's backed by Facebook and its support for static typing. There's some cons that come with the HHVM though, including not allowing custom extensions, that it's developed by Facebook (yes, this can be a con too) and that there are other ways to enhance PHP's execution speed without sacrificing other functionality.