在OOP中的php uasort

Question

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;
    }
}

I get the following warning :

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

And the 554th line contains uasort($posts_array, $this->cmp).

Where to make use of the string or array and in what way?

EDIT : If I use uasort($posts_array, array($this, 'cmp'));, I get the following warning:

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

Answer 1

If you have >= 5.3 and you're not using the compare method in any other function, you can also use closures:

uasort($posts_array, function($a, $b) {
    if ($a == $b) {
        return 0;
    }

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