Why do I recommend using client-side JS for templating? You will be using the client's browser power to generate html instead of your own. You free up bandwidth and compute power for other responses or things you want to do. Example, let's say you wanted to create a list on the page, well leave it for js! Let JS and CSS handle the templating for you. PHP sends the list data in a JSON format, we use JS to read that list and format it into html and attach whatever classes we need, then use CSS to style it up. Benefit: js files are cached! No need to keep re-sending html.
Serving data to the client and leaving templating as a client responsibility makes sense especially when you want to create a mobile application. You'll design your mobile app with your API in mind. Your API will probably have to detect if a mobile browser is requesting data or if the request contains some type of header indicating the need for data for a mobile app, then you can send the data that you need to send.
Am I supposed to show you a how to? If you understand this, go implement it now!