Okay, this is a bit of a personal recommendation, but I've had success with the free Palapa Web Server (edit: doesn't seem to be on the playstore anymore, but here's the creator's website in case it helps you find it), which is essentially a web / mysql server which runs on Android, which will run your PHP scripts, and it will also work offline.
To use it, install the app, and copy your PHP / HTML files onto your phone. By default, the app uses /sdcard/pws/www/, so if you put your files there, it should pick them up. Then, launch the app, click "Start server", and go to http://127.0.0.1:8080 with your web browser on your android device, and it should work.
Note that setting up PHPMyAdmin (if you need that for managing databases) can be a bit tricky, as you will need to download the extra packages for it.
Let me know if it works for you. (Note: I'm not affiliated with this app, I'm just a mostly satisfied user.)
