On Windows, this may be achieved using a similar approach to the ones mentioned in other replies.
In your /path/to/composer/phpunit directory, open the phpunit file in an editor. The first line should look like this:
#!/usr/bin/env php
Simply download your desired version of PHP for Windows, place the contents of the ZIP file somewhere to your liking, and reference the fully quantified path to the php.exe file, instead of just php. Like so:
#!/usr/bin/env /c/misc/php-5.5.9-nts-Win32-VC11-x86/php.exe
In my case, I put it in /c/misc/php-5.5.9-nts-Win32-VC11-x86/, which corresponds to C:\misc\php-5.5.9-nts-Win32-VC11-x86\ using Windows path syntax.
Remember to verify that the correct php.ini file is being used (php --ini or in a script file php_ini_loaded_file()).