I have a portable installation of XAMPP set up. Its php.exe is in my PATH. When I attempted to run Composer-Setup.exe, it correctly found php.exe, but then gave this error:

As far as I can tell, php.exe is fine - most of Composer's manual installation instructions worked, as I described in composer/composer#8883, and I got this output from running php.exe --version:

PHP 7.4.5 (cli) (built: Apr 14 2020 16:17:34) ( ZTS Visual C++ 2017 x64 )
Copyright (c) The PHP Group
Zend Engine v3.4.0, Copyright (c) Zend Technologies