Being skeptical is a good attitude. I'm very skeptical of this Eldorado that some posters see in PHP7. My experience is quite different.
> Additionally PHP has an amazing performance debugger by FB called xhprof.
xhprof is unmaintained for years. The official version does not compile with PHP7. Various forks exist, but the only stable fork has been rebranded and defaults to sending all the performance data to the branding company.
> PHP performance could always be increased hugely by making sure is cache buckets have enough memory (opcode cache, realpath_cache_size).
Always, really? The history of PHP opcode's caches is complex. Before PHP5.6 where Zend published their opcache+, I've seen the various solutions (APC and others) cause vicious bugs.
> Secondly with any web framework your app will most likely be limited by IO (database, file lookups, networking) before it becomes limited by actual code execution performance.
I've seen quite a few PHP applications that were CPU-bound, and that were far from Facebook's scale. For instance, the learning platform Moodle is popular with universities, but getting it to handle hundreds of concurrent users cannot be done on a plain quad-core server.
Another poster suggested load balancing as an obvious solution, but adding this is not that easy. E.g. Moodle has to track various files, including uploads. So once you add a load balancer and several PHP servers, you need to use a network mount for most of your files, which has a big impact on IO performance.
I'm not saying that other languages are better, because I can't compare the exact same large application in two languages, but PHP7 isn't an Eldorado.