Any other better logic for this programme - php

Question

Today in an interview i got the following question to solve without using any inbuilt function eg in_array and etc.I am able to solve the programme but they told me is there any better approach and also they told me the total code is only upto 7 lines.So can anyone tell me any better approach than this:-
<?php
$a = array(1,3,5,2,1,5,11,16);
$b = array(1,4,3,11,12,5,7,18);
$final = [];
for($i=0;$i<count($a);$i++){
$flag = 0;
if($i==0){
$final[] = $a[$i];
} else {
for($j=0;$j<count($final);$j++){
if($a[$i] == $final[$j]){
$flag = 1;
}
}
if($flag==0){
$final[] = $a[$i];
}
for($k=0;$k<count($final);$k++){
if($b[$i] == $final[$k]){
$flag = 1;
}
}
if($flag==0){
$final[] = $b[$i];
}
}
}
echo '<pre>';
print_r($final);
the result would be the final array which would contain unique array of both eg
Array ( [0] => 1 [1] => 3 [2] => 4 [3] => 5 [4] => 2 [5] => 11 [6] => 16 [7] => 18 )

Answer 1

Here is how you could do it without using built-in functions: Collect the values as keys (so they are unique), and then put those keys back as values in the final array:
$a = array(1,3,5,2,1,5,11,16);
$b = array(1,4,3,11,12,5,7,18);
foreach($a as $v) $keys[$v] = 1;
foreach($b as $v) $keys[$v] = 1;
foreach($keys as $k => $v) $final[] = $k;
echo '<pre>';
print_r($final);
See it run on eval.in.

Answer 2

Just set the key as the value and they will overwrite. If the arrays are the same length:
foreach($a as $k => $v) {
$final[$v] = $v;
$final[$b[$k]] = $b[$k];
}
If not, then do it for each array:
foreach(array($a, $b) as $c) {
foreach($c as $v) {
$final[$v] = $v;
}
}
The order and keys won't be the same, but that wasn't stated as a requirement.