This site has been acquired by Toptal.com.
(Attention! API endpoint has changed)

CSS Minifier

Online CSS Minifier/Compressor. Free! Provides an API. Simple Quick and Fast.

PHP

<?php
    // setup the URL and read the CSS from a file
    $url = 'https://www.toptal.com/developers/cssminifier/raw';
    $css = file_get_contents('styles.css');

    // init the request, set various options, and send it
    $ch = curl_init();

    curl_setopt_array($ch, [
        CURLOPT_URL => $url,
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_POST => true,
        CURLOPT_HTTPHEADER => ["Content-Type: application/x-www-form-urlencoded"],
        CURLOPT_POSTFIELDS => http_build_query([ "input" => $css ])
    ]);

    $minified = curl_exec($ch);

    // finally, close the request
    curl_close($ch);

    // output the $minified css
    echo $minified;
?>

Thanks to Boudewijn van Breukelen for an updated version of the above code.

Also, Toni Almeida has made a PHP library to easily talk to CSS Minifier : https://github.com/promatik/PHP-JS-CSS-Minifier.

Click on the language of your choice to see an example: