// use an array to mimic the behavior of a set
$set = [1, 2, 3, 4, 5];
$newElement = 4;
if (!in_array($newElement, $set)) {
$set[] = $newElement;
}
$set[] = 5;
print_r($set);
$set = array_unique($set);
print_r($set);
/*
run:
Array
(
[0] => 1
[1] => 2
[2] => 3
[3] => 4
[4] => 5
[5] => 5
)
Array
(
[0] => 1
[1] => 2
[2] => 3
[3] => 4
[4] => 5
)
*/