WTF: Refactoring snippet of the day

<?php
class SortingClassNameHere {
    public function __construct($string) {
        $this->string = $string;
    }

    private static $c = null;

    public static function cmp($a, $b) {
        $r = strnatcasecmp($a[self::$c], $b[self::$c]);
        return ($r > 0 ? 1 : ($r < 0 ? -1 : 0));
    }

    public function process($data) {
        self::$c = $this->string;
        if (!usort($data, array("SortingClassNameHere", "cmp"))) {
            throw new Exception('Unable to sort results.');
        }

        return $data;
    }
}

Hints:
* SortingClassNameHere::cmp() is never called anywhere else in the code base apart from process()
* If you don't know why this is bad, I will shoot you.