- var_dump: documentation ( source)
- array_merge: documentation ( source)
- array_keys: documentation ( source)
<?php
$arr1 = ["apple" => "bla", "peer" => "bleh"];
$arr2 = ["apple" => "foo", "bar" => "baz"];
$arr1Keys = array_keys($arr1);
$arr2Keys = array_keys($arr2);
$totalKeys = array_merge($arr1Keys, $arr2Keys);
$totalKeys = $arr1Keys + $arr2Keys;
var_dump($arr1Keys, $arr2Keys, $totalKeys);