Installing PHP on Windows - BCLibCoop/nnels-a11y-publishing GitHub Wiki
Instructional Video
https://app.box.com/s/ch7nqbbeuw4bqlbs5k9a71t9n85jd9fe
Written instructions
On Windows, you will need to download one or other version of PHP, install it, and then configure the DropToScript app so it knows where to find the PHP program.
There are many ways to achieve this; below the steps I used on Windows 10 to install the app.
Downloading PHP
Visit the PHP download site:
https://www.php.net/downloads.php
Pick a recent version (e.g. I picked PHP 7.3.4), then click 'Windows Downloads'. At the time of this writing it looked like this:

As far as I know any of the versions will work; I picked the 64-bit thread safe version.
Download the Zip, and decompress it. At the time of this writing it looked like this:

Unzip the downloaded archive, and move the whole unzipped directory to a convenient location.
I moved it into the 'Documents' directory.

I now have php.exe on my computer.

In my case, the full path is
C:\Users\Kris Coppieters\Documents\php-7.3.4-Win32-VC15-x64\php.exe
Optional: configure your path to include the PHP directory
You can optionally add the path to the php.exe file to your system settings.
Make sure you remember you did this: when you upgrade PHP some time in the future, you'll have to adjust this if the new PHP directory path is different than the previous one.
That will allow you to execute PHP from the command line by merely typing php instead of C:\Users\Kris Coppieters\Documents\php-7.3.4-Win32-VC15-x64\php or similar.
Bring up your control panel, and go into System and Security

Go into System

Click Advanced system settings at the left

Click Environment Variables

Double-click the line for Path

Click 'New' and on the new line, copy-paste the path to the directory that contains php.exe. In my case that path is C:\Users\Kris Coppieters\Documents\php-7.3.4-Win32-VC15-x64
This is the path of the parent directory, not the path for php.exe itself.

Click OK a few times to close all dialogs, and close the Control Panel windows that were left open.