PHP array_merge()

$d = array(
  'a' => 111,
  'b' => 222,
);

$e = array(
  'a' => 123,
  'c' => 333,
);
var_dump(array_merge($d, $e));

result:
array(3) {
  ["a"]=>
  int(123)
  ["b"]=>
  int(222)
  ["c"]=>
  int(333)
}

//-------------------------------------

$d = array(
  0 => 111,
  1 => 222,
);

$e = array(
  9 => 123,
  7 => 333,
);
var_dump(array_merge($d, $e));


result: (numeric key will be rearrange)
array(4) {
  [0]=>
  int(111)
  [1]=>
  int(222)
  [2]=>
  int(123)
  [3]=>
  int(333)
}

//--------------------------------------

$d = array(
  '100' => 111,
  '200' => 222,
);

$e = array(
  '100' => 100,
  '300' => 333,
);
var_dump(array_merge($d, $e));

result .... ?