$relatedObjects = array(
        2	=> 'object 2',
	3	=> 'object 3',
	5	=> 'object 5'
);
$collection = array(
	3	=> 'new instanceof object 3',
	5	=> 'new instanceof object 5',
	8	=> 'object 8',
	10	=> 'object 10',
	15	=> 'object 15',
);

$_relatedObjects = $relatedObjects; // set default array
$_relatedObjects = $collection + $_relatedObjects;
ksort($_relatedObjects);
echo '$_relatedObjects = $collection + $_relatedObjects;' . "\n";
print_r($_relatedObjects);

echo "\n";

$_relatedObjects = $relatedObjects; // set default array
$_relatedObjects = array_diff_key($_relatedObjects, $collection) + $collection;
ksort($_relatedObjects);
echo '$_relatedObjects = array_diff_key($_relatedObjects, $collection) + $collection;' . "\n";
print_r($_relatedObjects);
$_relatedObjects = $collection + $_relatedObjects;
Array
(
    [2] => object 2
    [3] => new instanceof object 3
    [5] => new instanceof object 5
    [8] => object 8
    [10] => object 10
    [15] => object 15
)

$_relatedObjects = array_diff_key($_relatedObjects, $collection) + $collection;
Array
(
    [2] => object 2
    [3] => new instanceof object 3
    [5] => new instanceof object 5
    [8] => object 8
    [10] => object 10
    [15] => object 15
)