// Fetches the value of $_GET['id'] and returns 0 if it does not exist.
$id = $_GET['id'] ?? 0;
// This is equivalent to:
$id = isset($_GET['id']) ? $_GET['id'] : 0;
// Coalescing can be chained: this will return the first defined value out of
// $_GET['id'], $_POST['id'], and 0.
$id = $_GE...
