[WINDOWS 10]

Open Windows search menu and type "Edit the system environment variables"

Click on it and it will give you a menu with some System properties...

Click the bottom right button labeled Environment Variables...

Go to the section labeled "System Variables" on the bottom...

Scroll through until you see Path...

Click on Path and hit edit...

Add a new line that points to your PHP installation. In my case (and likely yours too):

C:\xampp\php

And then boom, you're done.