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 value) {
    if (false === array_key_exists(union)) {
        $union[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.

$arr1 += $arr2 is short for $arr1 = $arr1 + $arr2.

The + array operator does the following:

• Create a new array that contains all the elements of $arr1 and $arr2, except for the next condition.
• If both operands have elements with the same key, only the element of $arr1 will be present.
• The elements of $arr2 will be after those of $arr1.

This is different from array_merge, which:

• Creates a new array that contains all the elements of $arr1 and $arr2, except for the next condition.
• If both operands have elements with the same string key, only the element of $arr2 will be present.
• Elements with numeric keys will be renumbered from 0, starting with the elements of $arr1, and then moving to the elements of $arr2.
• The elements of $arr2 will be after those of $arr1, except the string elements, which will be in the position of the first array in which they appear.

Example:

<?php
$arr1 = array(1 => 'value1.1', 10 => 'value1.2', 's' => 'value1.s');
$arr2 = array(1 => 'value2', 2=> 'value2.2', 's' => 'value2.s');
var_dump(array_merge($arr1,$arr2));
$arr1 += $arr2;
var_dump($arr1);

Result (edited for clarity):

array(5) {
  [0]   => string(8) "value1.1"
  [1]   => string(8) "value1.2"
  ["s"] => string(8) "value2.s"
  [2]   => string(6) "value2"
  [3]   => string(8) "value2.2"
}
array(4) {
  [1]   => string(8) "value1.1"
  [10]  => string(8) "value1.2"
  ["s"] => string(8) "value1.s"
  [2]   => string(8) "value2.2"
}