"Core Developers
The PHP Foundation contracts 10 full-time and part-time engineers to maintain and develop the PHP language."
Not to mention all the volunteers.
There are many, people doing (good, valuable) things on PHP, however active people with deep understanding of the engine can be counted on less than a hand. (I contributed some small things to the engine and other parts in the past)
One of those people is Nikita Popov (nikic). PHP core development took a hard hit when he left to work on other projects at JetBrains.