What's the difference between array_merge and array + array?

Question

A fairly simple question. What\'s the difference between:

$merged = array_merge($array1, $array2);

and

$merged = $array1 +          


        

Answer 1

Yet another example (arrays without explicit keys; it's obvious regarding to how the operator + and array_merge work, but "obvious" things are simpler when seen ;))

$a = array('apple');
$b = array('orange', 'lemon');

echo '$a + $b = ';             print_r($a + $b);
echo 'array_merge($a, $b) = '; print_r(array_merge($a, $b));

will give:

$a + $b = Array
(
    [0] => apple
    [1] => lemon
)
array_merge($a, $b) = Array
(
    [0] => apple
    [1] => orange
    [2] => lemon
)

Answer 2

Please pay attention for another difference: the union (+) won't overwrite non-empty value with empty value (considering a same key), whereas array_merge will:

$a = array('foo' => 'bar');
$b = array('foo' => ''); // or false or 0

print_r($a+$b);
print_r(array_merge($a, $b);

Outputs :

Array
(
    [foo] => bar
)
Array
(
    [foo] => 0
)

Answer 3

I believe array_merge overwrites duplicate non_numeric keys while $array1 + $array2 does not.

Answer 4

array_merge() causes all numeric keys found in the input arrays to be reindexed in the resultant array. The union operator + does not cause a reindex.

Answer 5

Source: https://softonsofa.com/php-array_merge-vs-array_replace-vs-plus-aka-union/

Stop using array_merge($defaults, $options):

function foo(array $options)
{
   $options += ['foo' => 'bar'];

   // ...
}

Note: array_replace function exists since PHP5.3.

Answer 6

So apparently if you change the order both union and merge will do the same thing

$a = array('foo' => 'bar', 'x' => 'fromA');
$b = array('foo' => null, 'x' => 'fromB');

echo '$a+$b: ';
var_dump($a+$b);

echo '$b+$a: ';
var_dump($b+$a);

echo 'array_merge($a, $b): ';
var_dump(array_merge($a, $b));

echo 'array_merge($b, $a): ';
var_dump(array_merge($b, $a));

Outputs :

$a+$b: array(2) {
  ["foo"]=>
  string(3) "bar"
  ["x"]=>
  string(5) "fromA"
}
$b+$a: array(2) {
  ["foo"]=>
  NULL
  ["x"]=>
  string(5) "fromB"
}
array_merge($a, $b): array(2) {
  ["foo"]=>
  NULL
  ["x"]=>
  string(5) "fromB"
}
array_merge($b, $a): array(2) {
  ["foo"]=>
  string(3) "bar"
  ["x"]=>
  string(5) "fromA"
}

Keep in mind the order of the arrays.