(I remembered seeing some other fb engineering talk on youtube around the release of hhvm on similar themes, a lot of extolling the edit/save/shift-ctl-r workflow and so on, and tried to find that one, but this is what came up.)
Same with Ruby, was fun when there was a lot of people using the language/tooling but nowadays the action is moving to other languages that are "cool" (go, rust, haskell, ...)
As others commented here, PHP becomes a serious bottleneck as a project becomes more complex. In fact, to deal with complexity you will see PHP tackling the problems in a manner very similar to Java, but without being Java. So in the end you'd likely be better off with Java to begin with.
As for what is actually "cool" about PHP, I would say nothing, however, I have to admit it is easy to find people willing to work with PHP. So PHP is maybe "cool" like 'Friday' was cool with so many people, I don't know, never been part of the "cool" crowd.
Sure, it might not do them all 100% perfectly, but it does them well enough to get things done.