php array merge without array_merge

<?php
$array1 = array("color" => "red", 2, 4);
$array2 = array("a", "b", "color" => "green", "shape" => "trapezoid", 4);
$result = array_merge($array1, $array2);
print_r($result);
?>
Array
(
    [color] => green
    [0] => 2
    [1] => 4
    [2] => a
    [3] => b
    [shape] => trapezoid
    [4] => 4
)

$arr1 = array("color1" => "red", "color2" => "blue");
$arr2 = array("color1" => "black", "color3" => "green");
$arr3 = $arr1 + $arr2; //result is array("color1" => "red", "color2" => "blue", "color3" => "green");