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

Question

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?

Answer 1

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.

Answer 2

The best example I found for using this is in a config array.

$user_vars = array("username"=>"John Doe");
$default_vars = array("username"=>"Unknown", "email"=>"no-reply@domain.com");

$config = $user_vars + $default_vars;

The $default_vars, as it suggests, is the array for default values. The $user_vars array will overwrite the values defined in $default_vars. Any missing values in $user_vars are now the defaults vars from $default_vars.

This would print_r as:

Array(2){
    "username" => "John Doe",
    "email" => "no-reply@domain.com"
}

I hope this helps!

Answer 3

This operator takes the union of two arrays (same as array_merge, except that with array_merge duplicate keys are overwritten).

The documentation for array operators is found here.

Answer 4

Carefull with numeric keys, if they should be preserved or if you don't want to loose anything

$a = array(2 => "a2", 4 => "a4", 5 => "a5");
$b = array(1 => "b1", 3 => "b3", 4 => "b4");

union

print_r($a+$b);
Array
(
    [2] => a2
    [4] => a4
    [5] => a5
    [1] => b1
    [3] => b3
)

merge

print_r(array_merge($a, $b));
Array
(
    [0] => a2
    [1] => a4
    [2] => a5
    [3] => b1
    [4] => b3
    [5] => b4
)

Answer 5

The + operator produces the same results as array_replace(). However since the operator arguments are reversed, the ordering of the resulting array may also be different.

Expanding on another example from this page:

$array1 = array('one', 'two', 'foo' => 'bar');
$array2 = array('three', 'four', 'five', 'foo' => 'baz'); 

print_r($array1 + $array2);
print_r(array_replace($array2, $array1)); //note reversed argument order

outputs:

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

Answer 6

1. Array plus operation treats all array as assoc array.
2. When key conflict during plus, left(previous) value will be kept

I post the code below to make things clear.

$a + $b = array_plus($a, $b)

function array_plus($a, $b){
    $results = array();
    foreach($a as $k=>$v) if(!isset($results[$k]))$results[$k] = $v;
    foreach($b as $k=>$v) if(!isset($results[$k]))$results[$k] = $v;
    return $results;
}

Answer 7

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

That being said, we could think of the + operator as kinda redundant, since the same can be achieved with array_replace function.

However, there are cases, where it comes in handy: say you have an $options array being passed to a function/method, and there are also defaults to be used as fallback:

// we could do it like this
function foo(array $options)
{
   $defaults = ['foo' => 'bar'];
   
   $options = array_replace($defaults, $options);
 
   // ...
}
 
// but + here might be way better:
function foo(array $options)
{
   $options += ['foo' => 'bar'];
 
   // ...
}

Answer 8

It will append the new array to the previous.

Answer 9

$var1 = "example";
$var2 = "test";
$output = array_merge((array)$var1,(array)$var2);
print_r($output);

Array ( [0] => example [1] => test )