如何仅在有人成功提交表单时运行 PHP 邮件表单脚本

我在同一页面上有以下带有PHP的HTML表单代码。实际上，问题是当任何人打开网站时，它会自动运行PHP Mail()脚本并向我发送一封空邮件。

我希望它只有在有人成功提交表单时才有效。请帮忙。

<!--HTML FORM--><form method="post" name="contactform" action="<?php echo htmlspecialchars($_SERVER['PHP_SELF']); ?>" onsubmit="return validateform()">
                <input type="text" name="fullname" class="fullname" placeholder="Full Name*"
       onfocus="this.placeholder = '';" onblur="if (this.placeholder == '') {this.placeholder = 'Full Name*';}">
                <input type="text" name="email" class="email" placeholder="Email Address*"
       onfocus="this.placeholder = '';" onblur="if (this.placeholder == '') {this.placeholder = 'Email Address*';}">
                <textarea placeholder="Your Message*:" name="message" class="message"
       onfocus="this.placeholder = '';" onblur="if (this.placeholder == '') {this.placeholder = 'Your Message*';}"></textarea>
       <input type="submit" name="submit" value="Submit" />
        </form><!--/HTML FORM> <!--Contact Form PHP-->
<?php
 // define variables and set to empty values
 $fullname = $email = $message = "";
 $subject="Message from Website Visitor:".test_input($_POST["fullname"]);
 $to="camadhusudanmishra@gmail.com";
 $headers = "From:".test_input($_POST["email"]);
 if ($_SERVER["REQUEST_METHOD"] == "POST") {
 $fullname = test_input($_POST["fullname"]);
 $email = test_input($_POST["email"]);
 $message = test_input($_POST["message"]);}
 function test_input($data) {
   $data = trim($data);
   $data = stripslashes($data);
   $data = htmlspecialchars($data);
   return $data;}
 $message = str_replace("'n.", "'n..", $message);
 mail($to,$subject,$message,$headers);
?>
 <!--/Contact Form PHP-->

用于验证的 JS 代码：

    function formValidation() {
    var fullname = document.contactform.fullname;
    var email = document.contactform.email;
    var message = document.contactform.message;
    if(validateName(fullname))
    {
    if(validateEmail(email))
    {
      if(validateMessage(message))
      {
      }
    }
  }
  return false;
}
function validateName(fullname)
  {
    var letters = /^[A-Za-z]+$/;
    if(fullname.value.match(letters) && fullname.value.length >= 3 && fullname.value.length <= 30)
    {
      return true;
    }
    else {
      alert('Please enter a valid name');
      return false;
    }
  }
function validateEmail(email)
  {
    var mailformat = /^'w+(['.-]?'w+)*@'w+(['.-]?'w+)*('.'w{2,3})+$/;
    if(email.value.match(mailformat))
    {
      return true;
    }
    else
    {
      alert("You have entered an invalid email address!");
      return false;
    }
  }
function validateMessage(message)
  {
    if(message.value.length >=5 && message.value.length <=300)
    {
      alert('Form Succesfully Submitted');
      window.location.reload();
      return true;
    }
    else
    {
      alert("Please type a valid message.");
      return false;
    }
}