> how many interesting new ideas will you learn from studying PHP, versus [...] languages such as Javascript/Node

This is on point! Programming PHP is not about diving deep into interesting language constructs or creating beautifully structured software - PHP is about getting things done, about starting hurdle-free and progressing rapidly. It goes against some "good development practices" but it works, see for example the single-file website remoteok.io from Pieter Levels: https://twitter.com/levelsio/status/938707166508154880