Error messages don't display under corrensponding input field. Using HTML, PHP

Question

How do I display corrensponding error message under the input field instantly if user inputs invalid data. For example, user inputs invalid data into "name" field and error message instantly appears under the name field. Same for "email" and "message" fields. I'm using next HTML & PHP code:

<form method="POST" action="send-email.php">
  
  <input type="text" name="name" id="name" placeholder="Name*" required>
  <div class="error"><?php echo $nameErr ?></div>

  <input type="email" name="email" id="email" placeholder="Email*" required>
  <div class="error"><?php echo $emailErr ?></div>

  <textarea name="message" id="message" placeholder="Your Message*" required></textarea>
  <div class="error"><?php echo $messErr ?></div>

  <button type="submit" name="submit" id="submit" class="button">Start</button>
  
</form>

<?php
require "vendor/autoload.php";

use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\SMTP;

$nameSanitized = filter_var(trim($_POST['name']), FILTER_SANITIZE_STRING);
$emailSanitized = filter_var($_POST['email'], FILTER_SANITIZE_EMAIL);
$messageSanitized = filter_var(trim($_POST['message']), FILTER_SANITIZE_STRING);
$subject = "new message ";

$valid = true;
$emailErr = "";
$nameErr = "";
$messErr = "";

if (!preg_match('/^[\p{Latin}\p{Cyrillic} ]*$/u', $nameSanitized)) {
    $nameErr = "Only Latin and Cyrillic letters and white space allowed in the name field.\n";
    $valid = false;
}

if (empty($nameSanitized) || empty($messageSanitized)) {
    $messErr = "Name and message are required.\n";
    $valid = false;
}

if (!preg_match("/^[a-zA-Z0-9_]+@[a-zA-Z0-9_]+\.[a-zA-Z0-9_]+$/", $emailSanitized)) {
    $emailErr = "Email should contain only letters, numbers, and underscores.\n";
    $valid = false;
}

//only send if all validations passed:
if ($valid == true) {
    try {
        $mail = new PHPMailer(true);

        $mail->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("[email protected]", "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.

Answer 1

Put the HTML code in your else block

} else { ?>
 
<form method="POST" action="send-email.php">
  
  <input type="text" name="name" id="name" placeholder="Name*" required>
  <div class="error"><?php echo $nameErr ?></div>

  <input type="email" name="email" id="email" placeholder="Email*" required>
  <div class="error"><?php echo $emailErr ?></div>

  <textarea name="message" id="message" placeholder="Your Message*" required></textarea>
  <div class="error"><?php echo $messErr ?></div>

  <button type="submit" name="submit" id="submit" class="button">Start</button>
  
</form>

<?php }