Regarding array union:  Here is a faster version array_union($a, $b)

But it is not needed!  See below.

<?php
                                        //  $a = 1 2 3 4
    $union =                            //  $b =   2   4 5 6
        array_merge(
            $a,    
            array_diff($b, $a)          //               5 6
        );                              //  $u = 1 2 3 4 5 6
?> 

You get the same result with $a + $b.

N.B. for associative array the results of $a+$b and $b+$a are different, I think array_diff_key is used.

Cheers, E