<?php

$arr1 = array(0=>'hello', 1=>'stack', 2=>'overflow');

$arr2 = array('key'=>'value', 5=>'value 2', 'something'=>'more value');

$arr3 = $arr1 + $arr2;

print_r($arr3);

Array
(
    [0] => hello
    [1] => stack
    [2] => overflow
    [key] => value
    [5] => value 2
    [something] => more value
)