PHP Deep Extend Array

How can I do a deep extension of a multi dimensional associative array (for use with decoded JSON objects). I need the php equivalent of jQuery's $.extend(true, array1, array2) with arrays instead of JSON and in PHP.

Here's an example of what I need (array_merge_recursive didn't seem to do the same thing)

$array1 = ('1'=> ('a'=>'array1a', 'b'=>'array1b'));
$array2 = ('1'=> ('a'=>'array2a', 'c'=>'array2b'));

$array3 = array_extend($array1, $array2);

//$array3 = ('1'=> ('a'=>'array2a', 'b'=>'array1b', 'c'=>'array2b'))

Notice how array2 overrides array1 if it has same value (like how extension of classes works)