PHP让数组中有相同值的组成新的数组

$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')
);

$result = array();
foreach ($arr as $data) {
isset($result[$data['key1']]) || $result[$data['key1']] = array();
$result[$data['key1']][] = $data['key2'];
}
ksort($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
)

)