Adding tools to your toolbelt is part of the craft, no? You can add another language to your roster without abandoning PHP. I used to write PHP, and will write it happily after a refresher if I need to.

However, I learnt ton of languages after I learnt PHP. That shouldn't be an issue.

On your question, PHP is alive because of the same reasons why FORTRAN and even COBOL alive. It's useful.

If you're a freelancer you're subject to what your customer wants. If you built custom decks and people started to not like the materials that they bought from you and you constructed, you'd have to change materials to stay competitive.