<?php
$ar1 = [
0 => '1-0',
'a' => '1-a',
'b' => '1-b'
];
$ar2 = [
0 => '2-0',
1 => '2-1',
'b' => '2-b',
'c' => '2-c'
];
Standard input is empty
Array
(
    [0] => 1-0
    [a] => 1-a
    [b] => 2-b
    [1] => 2-0
    [2] => 2-1
    [c] => 2-c
)
Array
(
    [0] => 1-0
    [a] => 1-a
    [b] => 1-b
    [1] => 2-1
    [c] => 2-c
)