How can I merge PHP arrays?

I have two arrays of animals (for example).
$array = array(
    array(
        'id' => 1,
        'name' => 'Cat',
    ),
    array(
        'id' => 2,
        'name' => 'Mouse',
    )
);

$array2 = array(
    array(
        'id' => 2,
        'age' => 321,
    ),
    array(
        'id' => 1,
        'age' => 123,
    )
);
How can I merge the two arrays into one by the ID?

Answer:

This does what Erik suggested (id no. as array key) and merges vlaues in $array2 to $results.
$results = array();

foreach($array as $subarray)
{
    $results[$subarray['id']] = array('name' => $subarray['name']);
}

foreach($array2 as $subarray)
{
    if(array_key_exists($subarray['id'], $results))
    {
        // Loop through $subarray would go here if you have extra 
        $results[$subarray['id']]['age'] = $subarray['age'];
    }
}