Arrays '+' VS Yii2 ArrayHelper::merge

php.php

<?php
// $arr1
Array
(
    [a] => 1
    [b] => 2
    [c] => 3
)

// $arr2
Array
(
    [a] => 11
    [b] =>
    [c] => 3
    [d] => 4
)
// $arr1 + $arr2
Array
(
    [a] => 1
    [b] => 2
    [c] => 3
    [d] => 4
)
// ArrayHelper::merge($arr1, $arr2)
Array
(
    [a] => 11
    [b] =>
    [c] => 3
    [d] => 4
)

// ArrayHelper::merge($arr2, $arr1)
Array
(
    [a] => 1
    [b] => 2
    [c] => 3
    [d] => 4
)