Create a directory for PHP binaries; e.g., C:\bin

Append ;C:\bin to your PATH environment variable (related help)

Download https://phar.phpunit.de/phpunit.phar and save the file as C:\bin\phpunit.phar

Open a command line (e.g., press Windows+R » type cmd » ENTER)

Create a wrapping batch script (results in C:\bin\phpunit.cmd):

C:\Users\username> cd C:\bin

C:\bin> echo @php "%~dp0phpunit.phar" %* > phpunit.cmd

C:\bin> exit

Open a new command line and confirm that you can execute PHPUnit from any path:

C:\Users\username> phpunit --version

PHPUnit x.y.z by Sebastian Bergmann and contributors.