This is my experience benchmarking the same as well. That, combined with a bug in SPLFixedArray that results in it acting like a non-fixed array past some size,

This is my experience benchmarking the same as well. That, combined with a bug in SPLFixedArray that results in it acting like a non-fixed array past some size, makes it very difficult to consider PHP for anything that requires high performance on tight loops/data crunching. To the extent that it is often viable to run another programming language's implementation of your code operating as a service for your PHP code.