## keys
獲取所有的鍵
例子：
~~~
$members=[
['id'=>1,'name'=>'小明','sex'=>'男','age'=>18,'register_time'=>'2017-04-01 08:30:01'],
['id'=>2,'name'=>'小紅','sex'=>'女','age'=>18,'register_time'=>'2017-04-02 12:21:32'],
['id'=>3,'name'=>'小剛','sex'=>'男','age'=>16,'register_time'=>'2017-04-05 09:10:13'],
['id'=>4,'name'=>'小麗','sex'=>'女','age'=>19,'register_time'=>'2017-04-05 12:05:27'],
['id'=>5,'name'=>'小壯','sex'=>'男','age'=>21,'register_time'=>'2017-04-08 15:41:59'],
['id'=>6,'name'=>'小東','sex'=>'男','age'=>23,'register_time'=>'2017-04-01 08:30:01'],
];
$dataSource=Traversable::from($members);
$result=$dataSource->keys();
foreach ($result as $s){
dump($s);
}
~~~
打印結果：
~~~
0
1
2
3
4
5
~~~
