Åben php error beskeder i en div-boks, frem for nyt vindue

Som en del af en email-sender fra en hjemmeside har jeg koden forneden.

Hvordan får jeg error-beskederne til at vise sig i en <div> boks ved siden af mine "forms" (som jeg har i et andet dokument), frem for at beskederne åbner sig i et helt nyt vindue?

<?php
if(isset($_POST['Email_Address'])) {
   
    include 'lite_settings.php';
   
    function died($error) {
        echo "Sorry, but there were error(s) found with the form you submitted. ";
        echo "These errors appear below.<br /><br />";
        echo $error."<br /><br />";
        echo "Please go back and fix these errors.<br /><br />";
        die();
    }
   
    if(!isset($_POST['Full_Name']) ||
        !isset($_POST['Email_Address']) ||
        !isset($_POST['Telephone_Number']) ||
        !isset($_POST['Your_Message'])) {
        died('We are sorry, but there appears to be a problem with the form you submitted.');       
    }
   
    $full_name = $_POST['Full_Name']; // required
    $email_from = $_POST['Email_Address']; // required
    $telephone = $_POST['Telephone_Number']; // not required
    $comments = $_POST['Your_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)==0) {
      $error_message .= 'The Email Address you entered does not appear to be valid.<br />';
  }
  if(strlen($full_name) < 2) {
      $error_message .= 'Your Name does not appear to be valid.<br />';
  }
  if(strlen($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.\r\n";
   
    function clean_string($string) {
      $bad = array("content-type","bcc:","to:","cc:","href");
      return str_replace($bad,"",$string);
    }
   
    $email_message .= "Full Name: ".clean_string($full_name)."\r\n";
    $email_message .= "Email: ".clean_string($email_from)."\r\n";
    $email_message .= "Telephone: ".clean_string($telephone)."\r\n";
    $email_message .= "Message: ".clean_string($comments)."\r\n";
   
$headers = 'From: '.$email_from."\r\n".
'Reply-To: '.$email_from."\r\n" .
'X-Mailer: PHP/' . phpversion();
@mail($email_to, $email_subject, $email_message, $headers);
header("Location: $thankyou");
?>
<script>location.replace('<?php echo $thankyou;?>')</script>
<?php
}
die();
?>