There is a pear package called Services_W3C_CSSValidator which does this.
You can download the PHP class directly from the github if you prefer.
Its very simple to use.
require_once 'Services/W3C/CSSValidator.php';
$v = new Services_W3C_CSSValidator();
$result = $v->validateFile('pear_manual.css'); // XML
It includes all features available at http://jigsaw.w3.org/css-validator