PHP の usort() 関数が不安定なソートである場合はどうすればよいですか?

<?php$a1 = array('key1'=>1,'key2'=>1);$a2 = array('key1'=>2,'key2'=>2);$a3 = array('key1'=>2,'key2'=>3);$a = array($a3,$a2,$a1);usort($a,"my_sort2");print_r($a);usort($a,"my_sort1");print_r($a);function my_sort1($a,$b){if ($a['key1']==$b['key1']) return 0;return ($a['key1']<$b['key1'])?-1:1;}function my_sort2($a,$b){if ($a['key2']==$b['key2']) return 0;return ($a['key2']<$b['key2'])?-1:1;}?>

Array(    [0] => Array        (            [key1] => 1            [key2] => 1        )    [1] => Array        (            [key1] => 2            [key2] => 2        )    [2] => Array        (            [key1] => 2            [key2] => 3        ))Array(    [0] => Array        (            [key1] => 1            [key2] => 1        )    [1] => Array        (            [key1] => 2            [key2] => 3        )    [2] => Array        (            [key1] => 2            [key2] => 2        ))

$a1 = array('key1'=>1,'key2'=>1);$a2 = array('key1'=>2,'key2'=>2);$a3 = array('key1'=>2,'key2'=>3);$a = array($a3,$a2,$a1);foreach($a as $v) {  $r2[] = $v['key2'];  $r1[] = $v['key1'];}array_multisort($r1,$r1, $a);print_r($a);

Array(    [0] => Array        (            [key1] => 1            [key2] => 1        )    [1] => Array        (            [key1] => 2            [key2] => 2        )    [2] => Array        (            [key1] => 2            [key2] => 3        ))

<?php$a1 = array('key1'=>1,'key2'=>1);$a2 = array('key1'=>2,'key2'=>2);$a3 = array('key1'=>2,'key2'=>3);$a = array($a3,$a2,$a1);foreach($a as $v){     $ar1[] = $v['key1'];     $ar2[] = $v['key2'];}array_multisort($ar1,SORT_ASC,$ar2,SORT_ASC, $a);print_r($a);?>