Using HTTPS is probably the easiest and most hassle free (and secure) way to go. You'll still be vulnerable to very knowledgable hac, uhm, pen-testers, but considering the rather benign nature of your traffic that risk is miniscule.
You can hop on over to https://letsencrypt.org/ where you can obtain a SSL or TSL certificate for free *and* have some very good documentation. As for the implementation itself, you need to read up on how to plug the certificate into your backend using your language of choice - PHP.
You can bake your own, it is easy enough but not by far as secure. Though it'll thwart your average script-kiddie using the standard Kali tools that is designed for your standard encryption regimes. I made one such just a few weeks ago. You can find that here: https://forum.thegamecreators.com/thread/219421