On the Dailymotion.com Engineering blog there's a recent post detailing their experiences moving their services to PHP 7 and some of the discoveries they made along the way.
In march 2015, we started to think that code refactoring and architecture improvements, will not be the only way to optimize the response time on dailymotion.com. This is the core problem of websites with high load : “how to scale without investing too much in people/servers”.
They started out by looking into Facebook's HHVM project to potentially replace the default PHP interpreter with a better performing core. They mention incompatibilities they discovered and some of the results in testing it on a handful of servers in production. They had some time to play with things so they waited until PHP 7 was officially released and tried that to make an equal comparison. In the end, they ultimately chose to go with PHP 7 as it was the route with less "friction" and work on changes for their current codebase. The post also includes graph output of some of the improvements they saw when