Well if you want to view the website on mobile rather you can view the pages on any resolution by using a simple shortcut "Ctrl + Shift +M". It is a developer tool called as the responsive design view.

PHP is a scripting language that is interpreted on the web server and delivered to the browser as HTML. The .php extension can be used with files that are mostly plain HTML anyway, or with files that are exclusively program code. But either way, you will have the best results viewing them on a web server instead of as locally stored files.

To answer your question: I don't know if there is an add-on to process PHP files and show them as HTML.

Thanks for that idea, Varun. Unfortunately, when I did CTRL+SHIFT+M on a webpage on the PC it gives a view that's very different to what I can see on my mobile phone. Firstly, although images reduce in proportion to the viewport, the font size remains fixed. Still, it's good to know this trick.

But either way, you will have the best results viewing them on a web server instead of as locally stored files.

Is this because web servers have .php file processing software on board ? Still, it's funny how Firefox Android refuses to display a .php file when you try to access it by the file opener, yet still displays it no problem when you follow a link inside an open html webpage . . .

Maybe this points to a problem with the Android OS's file associations rather than Firefox Android . . .

Initially I tried to open this offline webpage with the Samsung browser. (I believe that this is actually a version of Google Chrome, renamed "Internet".) But this seems to refuse all attempts to open a webpage when there is no data-link . . . So I had to install another browser like Firefox Android.

So maybe it really is a (Samsung) Android issue at heart.