$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);//這個是鍵值按字母先后順序排列 print_r($result);  //輸出如下 Array (   [value1] => Array     (       [0] => value2       [1] => value3     )    [value2] => Array     (       [0] => value4       [1] => value5     )      )