If the user enters invalid data, how to immediately display the corresponding error message under the input field. For example, the user enters invalid data in the Name field and an error message immediately appears under the Name field. The same goes for the Email and Message fields. I'm using the next HTML and PHP code:

  
  
  
 Start 

SMTPDebug = SMTP::DEBUG_SERVER; $mail->isSMTP(); $mail->SMTPAuth = true; $mail->Host = "smtp.gmail.com"; $mail->SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS; $mail->Port = 587; require_once 'config.php'; $mail->Username = SMTP_USERNAME; $mail->Password = SMTP_PASSWORD; $mail->setFrom($emailSanitized, $nameSanitized); $mail->addAddress("myemail@gmail.com", "myemail"); $mail->Subject = $subject; $mail->Body = "Name: $nameSanitized\nEmail: $emailSanitized\n\n$messageSanitized"; $mail->send(); header("Location: sent.html"); } catch (Exception $e) { echo 'Message could not be sent. Mailer Error: ', $mail->ErrorInfo; } } else { echo $emailErr; echo $nameErr; echo $messErr; } ?>

I tried different articles.