This package is intened to be installed in the WordPress root and loaded via autoload.php so WordPress plays nicely with php -S.

composer require aubreypwd/php-s-wp

Then in your wp-config.php make sure and require autoload.php e.g.:

if ( file_exists( __DIR__ . '/vendor/autoload.php' ) ) {
	require_once __DIR__ . '/vendor/autoload.php';
}

Now you can start up php -S in your WordPress root and places like http://localhost:8000/wp-admin will be automatically re-directed to http://localhost:8000/wp-admin/ so things don't break.