<h2>Invalid access.</h2>session_start(); // Always at the top

// ... [input validation and PHPMailer setup]

if (count($errors) === 0) {
    $mail = new PHPMailer(true);
    try {
        // [PHPMailer SMTP setup, etc.]
        $mail->send();
        unset($_SESSION['phpmailer_debug']);
        header("Location: index.php?success=1");
        exit();
    } catch (Exception $e) {
        $_SESSION['form_errors'] = ["Could not send email. Mailer Error: {$mail->ErrorInfo}"];
        // Add SMTP debug trace:
        if (isset($_SESSION['phpmailer_debug'])) {
            $_SESSION['form_errors'][] = implode("<br>", $_SESSION['phpmailer_debug']);
            unset($_SESSION['phpmailer_debug']);
        }
        header("Location: index.php?success=0");
        exit();
    }
} else {
    $_SESSION['form_errors'] = $errors;
    header("Location: index.php?success=0");
    exit();
}