In command prompt type below

set PATH=%PATH%;C:\path\to\php

Where C:\path\to\php is the folder where your php.exe file is located. After this run

C:\>php -v

than you will see something like

PHP 7.3.6 (cli) (built: May 29 2019 12:11:00) 

UPDATE:

If you find that when you restart your computer, this solution will not work anymore:

Set the Path like as following:

Step 1 - Click on the Windows icon

Step 2 - Click on the Settings icon

Step 3 - Click on System

Step 4 - Click on About

Step 5 - Click on System info

Step 6 - Click on Advanced system settings

Step 7 - Click on Environment variables...

Step 8 - Select Path row and then click Edit

Step 9 - Click New and then click Browse, then in the next panel which will open you need to select the folder you want in the Path. For the initial premise of this guide i will add the folder C:\Users\NewFolderInPath

Note: Replace your PHP installed path(e.g:C:\Program Files\php) with the above path .

Step 10 - Click OK and click every OK button you will encounter to close every previous windows.

You can edit the Windows Path variable for PHP. To do so, right click on the Computer folder and choose "properties". That should open a Control Panel window. In that window, click on "Advanced system settings" in the panel on the left, and then click on the "Environment Variables" button. Then select "Path" and click on "Edit...". Here is a screenshot of these steps (sorry it's in Swedish but the buttons should be in the same place in any language):

On Windows 11 the first window looks different, there you can find "Advanced system settings" under "Device specifications" as shown below. The other windows look exactly the same.

When you've clicked on "Edit...", you will get a window with a list of a lot of folder paths like this:

In that list, locate the one that says C:\wamp\bin\php\php5.6.25 (or whatever version of PHP it uses by default), edit it by double-clicking on it, and replace it with whatever PHP version you want to use, for example C:\wamp\bin\php\php7.3.1 (You need to specify a version to PHP that is installed, to see which ones are installed, go to the folder C:\wamp\bin\php in Windows Explorer, each version of PHP that is installed has its own folder in there). When you're done editing the path variable, click on OK in all windows you opened and you should be done. If you had the command prompt open while you were doing this, you may need to restart the command prompt for it to take effect.