php(二维数组中按条件排序的一个小疑点)

//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));