The answer here is non-trivial. I have since long stopped developing with PHP, but PHP has several run modes (And multiple runtimes like HHVM), that may bring different performance and concurrency characteristics.
Last I remember, PHP still runs all inside of a single process, so it still all share the same memory space, and it no longer has the overhead of starting a new process with every request.
The piece of engineering that made node.js so fast back in the day was Libuv, which allowed for non-blocking IO, greatly reducing the number of system calls/context switches.
But I am also going to guess that PHP developers have since caught on to the performance optimizations of non-blocking IO, and integrated the improvements into the runtime. Doing a quick search for "php vs nodejs benchmark" on Google [1], it seems like the performance of Nodejs is comparable to that of HHVM.
So as usual, use the right tool for the job. This is more of a problem of using your tooling correctly, more than choosing the correct language.
Node.js uses non-blocking IO to achieve massive amounts of concurrency on a single thread, while one-thread-per-request models such as PHP rely on OS threading to do the exact same thing.
If anything, the node.js model is often capable of more concurrency because of the inefficiencies involved in OS threading and context switching.
EDIT: of course, event driven PHP exist. I don't know much about the current state of it.
