- var_dump: documentation ( source)
- array_diff: documentation ( source)
<?php
$a1 = array(1=>1, 2=>2, 3=>3, 4=>4, 5=>5, 6=>6, 7=>7); //refrenced elsewhere
$a2 = array(2=>2, 4=>4, 6=>6, 8=>8); //currently exist
$a3 = array_diff($a1, $a2);
var_dump($a3);