PHP is rarely the bottleneck.
I've never thought of PHP as more than a simple tool to solve problems.
All the PHP code I've seen in that experience has been messy, unmaintainable crap. Spaghetti SQL wrapped in spaghetti PHP wrapped in spaghetti HTML, replicated in slightly-varying form in dozens of places.
PHP is about as exciting as your toothbrush. You use it every day, it does the job, it is a simple tool, so what? Who would want to read about toothbrushes?