if any of the keys of the first level of arrays are
numeric, using array_merge_recursive() will renumber
them to start at 0. this destroys keys that are used
to record id numbers, etc.

[mini:pgl]:~/public_html $ cat test.php 
#!/usr/local/bin/php -q
<?
$b[a][4] = 1;
$a[3][4] = 2;
print_r(array_merge_recursive($a, $b));
?>

[mini:pgl]:~/public_html $ ./test.php 
Array
(
    [0] => Array
        (
            [4] => 2
        )

    [a] => Array
        (
            [4] => 1
        )
)