如何在Zend Action Helper（或类中）中使用uasort（）？

I want to sort an array by using uasort() function. I wrote this piece of code and it worked well:

    function cmp($a, $b)
    {
        $_a = strlen($a);
        $_b = strlen($b);
        if($_a==$_b)
        {
            return 0;
        }

        return ($_a < $_b)? -1 : +1;
    }

    $arr = array(
            "234560"=>"the quick brown fox jump",
            "234561"=>"the quick brown fox jump over the lazy dog",
            "234562"=>"the quick brown"
        );

    $array = uasort($arr,"cmp");

But when I put it in a class, it return null with a warning:

Warning: uasort() expects parameter 2 to be a valid callback, function 'cmp' not found or invalid function name in...

Here is my code

class Zend_Controller_Action_Helper_Utility extends Zend_Controller_Action_Helper_Abstract
{

    public function uasort($array)
    {
        return uasort($array, "cmp");
    }

    function cmp($a, $b)
    {
        $_a = strlen($a);
        $_b = strlen($b);
        if ($_a == $_b)
        {
            return 0;
        }

        return ($_a < $_b) ? -1 : +1;
    }

}

I also try

return uasort($array, "Zend_Controller_Action_Helper_Utility::cmp");

with static function cmp(){} but I still return null.

How can I fix it?

You are specifying the callback wrong. If it is an instance method, you need to specify it as

array($this, 'cmp');

If you make it a static method, you need to specify it as

array('Zend_Controller_Action_Helper_Utility', 'cmp');

See the documentation of callback for more details.