I wasn't happy with the performance of the existing PHP port of JSMin, so I wrote my own implementation, which is significantly smaller and faster.
This new library will be included in the next release of Minify, and you're welcome to download it and use it in your own projects:
require 'jsmin.php';
// Output a minified version of example.js.
echo JSMin::minify(file_get_contents('example.js'));