PHP append one array to another (not array_push or +)

How to append one array to another without comparing their keys?

$a = array( 'a', 'b' );
$b = array( 'c', 'd' );

At the end it should be: Array( [0]=>a [1]=>b [2]=>c [3]=>d )
If I use something like [] or array_push, it will cause one of these results:

Array( [0]=>a [1]=>b [2]=>Array( [0]=>c [1]=>d ) )
//or
Array( [0]=>c [1]=>d )

It just should be something, doing this, but in a more elegant way:

foreach ( $b AS $var )
    $a[] = $var;