(PECL ds >= 1.0.0)
Ds\Map::union — Creates a new map using values from the current instance and another map
    Creates a new map that contains the pairs of the current instance as well as the
    pairs of another map.
  
    A ∪ B = {x: x ∈ A ∨ x ∈ B}
  
注意:
Values of the current instance will be overwritten by those provided where keys are equal.
mapThe other map, to combine with the current instance.
    A new map containing all the pairs of the current instance as well as another map.
    
示例 #1 Ds\Map::union() example
<?php
$a = new \Ds\Map(["a" => 1, "b" => 2, "c" => 3]);
$b = new \Ds\Map(["b" => 3, "c" => 4, "d" => 5]);
print_r($a->union($b));
?>
以上例程的输出类似于:
Ds\Map Object
(
    [0] => Ds\Pair Object
        (
            [key] => a
            [value] => 1
        )
    [1] => Ds\Pair Object
        (
            [key] => b
            [value] => 3
        )
    [2] => Ds\Pair Object
        (
            [key] => c
            [value] => 4
        )
    [3] => Ds\Pair Object
        (
            [key] => d
            [value] => 5
        )
)