class DBNews {

    public function get_latest_posts($limit){

        // code goes here

        $posts_array = array();
        uasort($posts_array, $this->cmp);

    }

    public function cmp($a, $b) {
        if ($a == $b) {
            return 0;
        }

        return ($a < $b) ? -1 : 1;
    }
}

Eu recebo o seguinte aviso:

Warning: uasort() expects parameter 2 to be a valid callback, 
no array or string given in 
C:\xampp\htdocs\news\admin\functions.php on line 554.

E a linha 554 contémuasort($posts_array, $this->cmp).

Onde usar a string ou array e de que maneira?

EDITAR : Se eu usaruasort($posts_array, array($this, 'cmp'));, Recebo o seguinte aviso:

uasort() expects parameter 2 to be a valid callback,
array must have exactly two members in
C:\xampp\htdocs\news\admin\functions.php on line 554