On the Symfony Finland blog there's a new post looking ahead at what they see in store for PHP's performance in a post-PHP 7.0.x world.
PHP 7.0 made significant improvements in terms of performance and memory use for real applications. Many applications deliver twice the throughput with much less memory just without any changes to the application code.
But with networked API driven architectures individual response times are increasingly critical for end-user experience. Luckily, there are quite a few unbeaten paths for regarding PHP performance.
These other "unbeaten paths" they mention include a trend towards using asynchronous patterns and the use of application servers (long running PHP processes). There's also mentions of JIT (just in time) compilation, defined request and response objects and the improvement of other possible PHP runtimes.