You can use this plugin:
https://github.com/promatik/PHP-JS-CSS-Minifier
It uses this minifier http://javascript-minifier.com/
Or use this script from
http://javascript-minifier.com/examples
<?php
// setup the URL, the JS and the form data
$url = 'http://javascript-minifier.com/raw';
$js = file_get_contents('./public/ready.js');
$data = array(
'input' => $js
);
// init the request, set some info, send it and finally close it
$ch = curl_init($url);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$minified = curl_exec($ch);
curl_close($ch);
// output the $minified
echo $minified;
?>
Don't forget to cache everything :)