It’s much the same whatever you use. Chop your site up into generic components (header, nav, footer etc). Reuse those. If your just building websites, by all means stick with PHP - you’re just rendering static pages so it’ll work great for that.
Re browser testing, yes, but it costs $$$ if you don’t want to keep signing up for a free trial period under different emails every hour (Browserstack for example). Normally very much cheaper to manually test in browsers, only costs time. Test ideally in Chrome, Firefox, IE/Edge, Android and OSX. Chrome has a pretty good emulator for mobile devices, and is excellent for a debugging Android phones/tablets. If you’re on a Mac, Safari has excellent support for debugging iPhones/Pads and XCode has a pretty good device emulator.
Note that on a Mac, you need a VM to get IE/Edge, and if you’re on Windows you can’t get Safari.
Edit: re front end frameworks like React etc, if you’re building a traditional website you don’t really need to think about them. Multi page traditional websites are basically a solved problem, and PHP does just fine, with a sprinkling of JS for the interactive parts. But if you want to start thinking about removing things like actually opening a new page every time you navigate to a different part of the site, making it more app-like, that’s where they come in: doing stuff like that without a framework tends to get hairy and you end up with a big ball of JS mess.