如果在验证联系表单时发生任何错误，则应出现警告消息框

A alert box should appear if there occurs any error below while submitting the form. And there should also appear a message box if the form is submitted correctly. The existing code takes me to the other pages when there are any errors occured. And also takes me to the other page when the form is submitted. I need the messsage box for the errors occured accoring to the given below code.

Below is my Php code

<?php
if(isset($_POST['Email'])) {

    // CHANGE THE TWO LINES BELOW
    $email_to = "jay44556677@gmail.com";

    $email_subject = "website html form submissions";


    function died($error) {
        // your error code can go here
        echo "We're sorry, but there's errors found with the form you submitted.<br /><br />";
        echo $error."<br /><br />";
        echo "Please go back and fix these errors.<br /><br />";
        die();
    }

    // validation expected data exists
    if(!isset($_POST['Name']) ||
        //!isset($_POST['last_name']) ||
        !isset($_POST['Email']) ||
        //!isset($_POST['telephone']) ||
        !isset($_POST['Message'])) {
        died('We are sorry, but there appears to be a problem with the form you submitted.');       
    }

    $first_name = $_POST['Name']; // required
    //$last_name = $_POST['last_name']; // required
    $email_from = $_POST['Email']; // required
    //$telephone = $_POST['telephone']; // not required
    $comments = $_POST['Message']; // required

    $error_message = "";
    $email_exp = '/^[A-Za-z0-9._%-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}$/';
  if(!preg_match($email_exp,$email_from)) {
    $error_message .= 'The Email Address you entered does not appear to be valid.<br />';
  }
    $string_exp = "/^[A-Za-z .'-]+$/";
  if(!preg_match($string_exp,$first_name)) {
    $error_message .= 'The First Name you entered does not appear to be valid.<br />';
  }
  //if(!preg_match($string_exp,$last_name)) {
    //$error_message .= 'The Last Name you entered does not appear to be valid.<br />';
  //}
  if(strlen($comments) < 2) {
    $error_message .= 'The Comments you entered do not appear to be valid.<br />';
  }
  if(strlen($error_message) > 0) {
    died($error_message);
  }
    $email_message = "Form details below.

";

    function clean_string($string) {
      $bad = array("content-type","bcc:","to:","cc:","href");
      return str_replace($bad,"",$string);
    }

    $email_message .= "Name: ".clean_string($first_name)."
";
    //$email_message .= "Last Name: ".clean_string($last_name)."
";
    $email_message .= "Email: ".clean_string($email_from)."
";
    //$email_message .= "Telephone: ".clean_string($telephone)."
";
    $email_message .= "Comments: ".clean_string($comments)."
";


// create email headers
$headers = 'From: '.$email_from."
".
'Reply-To: '.$email_from."
" .
'X-Mailer: PHP/' . phpversion();
@mail($email_to, $email_subject, $email_message, $headers);  
?>

<!-- place your own success html below -->

Thank you for contacting us. We will be in touch with you very soon.

<?php
}
die();
?>

There are several ways - ajax (as Barmar suggested) or just regular javascript function called on the button click (onClick()).

If you choose the first method you can find multitude of script examples here or elsewhere on the Internet. If you choose the latter method, then don't use the <input type="submit" /> but <input type="button" onclick="yourfunction();return false" />. The last bit (return false) is actually quite important. Among others you can check this topic; it might give you some further ideas.