Merging two arrays with the "+" (array union operator) How does it work?

I have some code that appears to merge the data from two arrays using +=, but it doesn't include all of the elements in the element. How does it work?

Example:

$test = array('hi');
$test += array('test', 'oh');
var_dump($test);

Output:

array(2) {
  [0]=>
  string(2) "hi"
  [1]=>
  string(2) "oh"
}

What does + mean when used on arrays in PHP?

Quoting from the PHP Manual on Language Operators

The + operator returns the right-hand array appended to the left-hand array; for keys that exist in both arrays, the elements from the left-hand array will be used, and the matching elements from the right-hand array will be ignored.

So if you do

$array1 = ['one',   'two',          'foo' => 'bar'];
$array2 = ['three', 'four', 'five', 'foo' => 'baz']; 

print_r($array1 + $array2);

You will get

Array
(
    [0] => one   // preserved from $array1 (left-hand array)
    [1] => two   // preserved from $array1 (left-hand array)
    [foo] => bar // preserved from $array1 (left-hand array)
    [2] => five  // added from $array2 (right-hand array)
)

So the logic of + is equivalent to the following snippet:

$union = $array1;

foreach ($array2 as $key => $value) {
    if (false === array_key_exists($key, $union)) {
        $union[$key] = $value;
    }
}

If you are interested in the details of the C-level implementation head to

• php-src/Zend/zend_operators.c

---

Note, that + is different from how array_merge() would combine the arrays:

print_r(array_merge($array1, $array2));

would give you

Array
(
    [0] => one   // preserved from $array1
    [1] => two   // preserved from $array1
    [foo] => baz // overwritten from $array2
    [2] => three // appended from $array2
    [3] => four  // appended from $array2
    [4] => five  // appended from $array2
)

See linked pages for more examples.