Important Lesson on php's array union (+) operator and array_merge() function

array_merge.php

<?php

$a[0] = 1;
$a[1] = 2;
$a[2] = 3;
$b[0] = 'a';

$b[1] = 'b';
$b[2] = 'c';
$b[3] = 1;

print "a array: \n";
print_r($a);
print "b array: \n";
print_r($b);

// array union op, unions on key! not val!!
$plus = $a + $b;
print "array + \n";
print_r($plus);

$arr_merge = array_merge($a, $b);
print " array w merge\n";
print_r($arr_merge);

$arr_merge_uniq = array_unique(array_merge($a, $b));
print " array w merge and uniq\n";
print_r($arr_merge_uniq);

result.txt

a array: 
Array
(
    [0] => 1
    [1] => 2
    [2] => 3
)
b array: 
Array
(
    [0] => a
    [1] => b
    [2] => c
    [3] => 1
)
array + 
Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 1
)
 array w merge
Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => a
    [4] => b
    [5] => c
    [6] => 1
)
 array w merge and uniq
Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => a
    [4] => b
    [5] => c
)

Hello, with what version of PHP supports array + array ?