(PECL ds >= 1.0.0)
Ds\Map::keys — Returns a set of the map's keys.
Returns a set containing all the keys of the map, in the same order.
此函数没有参数。
A Ds\Set containing all the keys of the map.
Example #1 Ds\Map::keys() example
<?php
$map = new \Ds\Map(["a" => 1, "b" => 2, "c" => 3]);
var_dump($map->keys());
?>
以上例程的输出类似于：
object(Ds\Set)#2 (3) {
[0]=>
string(1) "a"
[1]=>
string(1) "b"
[2]=>
string(1) "c"
}