php(二维数组中按条件排序的一个小疑点)

php(二维数组中按条件排序的一个小问题)
//原数组
Array ( [1] => Array ( [date] => 2011-08-18 [num] => 5 ) [2] => Array ( [date] => 2011-08-20 [num] => 3 ) [3] => Array ( [date] => 2011-08-17 [num] => 10 ) ) 

//方法一、$ar = Array ( 1 => Array ( 'date' => '2011-08-18', 'num' => 5 ),             2 => Array ( 'date' => '2011-08-20', 'num' => 3 ),             3 => Array ( 'date' => '2011-08-17', 'num' => 10 )          );function mysort($a, $b) {   $a = strtotime($a['date']);   $b = strtotime($b['date']);   if ($a == $b) return 0;   return ($a < $b) ? -1 : 1;}usort($ar, 'mysort');print_r($ar);//方法二、$arr=array ( '1' => array ( 'date' => '2011-08-18', 'num' => 5 ) ,'2' => array ( 'date' => '2011-08-20', 'num' => 3 ) ,'3' => array ( 'date' => '2011-08-17', 'num' => 10 ) )  ; foreach($arr as $v){      $tmp[$v[date]]=$v; } ksort($tmp); print_r(array_values($tmp));