php isset — 检测变量是否设置 foreach循环运用

例子

$a = 336 ;
$b = 33 ;
function large($x,$y){
    if((!isset($x))||(!isset($y))){
       // echo "this function require two numbers";
      return false;
    }
    if($x>$y){
        return $x;
    }else{
        return $y;
    }
}
echo large($v,$b);

 假设当前数组为
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')
)
如何将这个array中 key1的value 相同的情况下 将key2的value合并成一个新的数组。求写一个方法将原数组转化问 如下数组
array(
0=>array('value1'=>array('value2','value3')),
1=>array('value2'=>array('value4','value5')),
...
)

题主，是无论 key1是否有重复都独立吗？下面是这样的做法

$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']][] = $data['key2'];
}
print_r($result);

//输出如下
Array
(
    [value1] => Array
        (
            [0] => value2
            [1] => value3
        )

    [value2] => Array
        (
            [0] => value4
            [1] => value5
        )
        
)