if (!$plan) {
    $default_plans = [
        1 => ['id'=>1,'name'=>'Monthly','price'=>9.99,'duration_days'=>30],
        2 => ['id'=>2,'name'=>'3 Months','price'=>24.99,'duration_days'=>90],
        3 => ['id'=>3,'name'=>'6 Months','price'=>44.99,'duration_days'=>180],
        4 => ['id'=>4,'name'=>'Yearly','price'=>79.99,'duration_days'=>365],
    ];
    if (isset($default_plans[$plan_id])) $plan = $default_plans[$plan_id];
    if (!$plan) { header("Location: /pricing"); exit; }
}

$price_info = calculate_discounted_price($plan["price"], $plan["duration_days"]);
$plan_price = $price_info["price"];
$plan_discount = $price_info["discount"];