Due to the ubiquity and ease of hosting PHP, and the current shift to full-stack JavaScript frameworks, I predict a long transition period where various "bridges" will be made to interface between the two. Mainly I'm thinking of PHP-side exposing content via REST APIs, with a more modern frontend written all in JS.
Where I work we use PHP 7.0/7.1 with the Laravel framework and Vue.js as the front end. Works well for us!
I'm curious, how do you reconcile server- and client-side rendering to be the same? Specifically, I'm thinking about initial page load/reload and route transitions. Do you share templates/partials between PHP and JavaScript?