(PECL ds >= 1.0.0)
Ds\Map::diff — Creates a new map using keys that aren't in another map.
$map
   )
    Returns the result of removing all keys from the current instance that are
    present in a given map.
  
    A \ B = {x ∈ A | x ∉ B}
  
mapThe map containing the keys to exclude in the resulting map.
    The result of removing all keys from the current instance that are
    present in a given map.
  
Example #1 Ds\Map::diff() example
<?php
$a = new \Ds\Map(["a" => 1, "b" => 2, "c" => 3]);
$b = new \Ds\Map(["b" => 4, "c" => 5, "d" => 6]);
var_dump($a->diff($b));
?>
The above example will output something similar to:
object(Ds\Map)#3 (1) {
  [0]=>
  object(Ds\Pair)#4 (2) {
    ["key"]=>
    string(1) "a"
    ["value"]=>
    int(1)
  }
}