如何使phpmailer代码只在表单填写和提交时执行？

<?php
if ($_SERVER["REQUEST_METHOD"] == "POST" && isset($_POST["submit"])) {
    // 表单提交时执行以下代码

    // 验证和过滤表单数据
    $name = $_POST["name"];
    $email = $_POST["email"];
    $message = $_POST["message"];

    // 执行phpmailer代码
    require 'path/to/PHPMailerAutoload.php';

    $mail = new PHPMailer;
    $mail->isSMTP();
    $mail->Host = 'smtp.example.com';
    $mail->SMTPAuth = true;
    $mail->Username = 'your-email@example.com';
    $mail->Password = 'your-password';
    $mail->SMTPSecure = 'tls';
    $mail->Port = 587;

    $mail->setFrom('your-email@example.com', 'Your Name');
    $mail->addAddress('recipient@example.com', 'Recipient Name');
    $mail->Subject = 'Subject of the email';
    $mail->Body = 'Content of the email';

    if (!$mail->send()) {
        echo 'Message could not be sent.';
        echo 'Mailer Error: ' . $mail->ErrorInfo;
    } else {
        echo 'Message has been sent.';
    }
}
?>