$arr = array( 
  0=>array('key1'=>'value1' , 'key2'=>'value2'), 
  1=>array('key1'=>'value1' , 'key2'=>'value3'), 
  2=>array('key1'=>'value2' , 'key2'=>'value4'), 
  999=>array('key1'=>'value2' , 'key2'=>'value5') 
); 
 
$result = array(); 
<span style="color:#FF0000;">foreach ($arr as $data) { 
  isset($result[$data['key1']]) || $result[$data['key1']] = array(); 
  $result[$data['key1']][] = $data['key2']; 
}</span> 
ksort($cityAr, SORT_NATURAL);//这个是键值按字母先后顺序排列 
print_r($result); 
 
//输出如下 
Array 
( 
  [value1] => Array 
    ( 
      [0] => value2 
      [1] => value3 
    ) 
 
  [value2] => Array 
    ( 
      [0] => value4 
      [1] => value5 
    ) 
     
)