$arr = array(

    '11'=>array(

        'a'=>1,

        'b'=>2,

    ),

    '22'=>array(

        'a'=>3,

        'b'=>4,

    ),

    '33'=>array(

        'a'=>5,

        'b'=>6,

    )

);

$data = $this->array_sort($arr,'a','desc','yes');

private function array_sort($arr,$keys,$orderby='asc',$key='no'){

    $keysvalue = $new_array = array();

    foreach($arr as $k=>$v){

        $keysvalue[$k] = $v[$keys];

    }

    if($orderby=='asc'){

        asort($keysvalue);

    }else{

        arsort($keysvalue);

    }

    reset($keysvalue);

    foreach($keysvalue as $k=>$v){

        if($key=='yes'){

            $new_array[$k] = $arr[$k];

        }else{

            $new_array[] = $arr[$k];

        }

    }

    return $new_array;

}