<?php 
//  require_once('Mail.php');
if (isset($_POST['contact_us']))
{
  unset($_POST['contact_us']);
 
  if( $_SESSION['code'] == $_POST['code'] )
  {
       if ( ($_POST['message'] !== strip_tags($_POST['message'])) || ($_POST['name'] !== strip_tags($_POST['name']))  )
        {
            $msg_danger = "Invalid input: HTML or script tags are not allowed..";
        }
        else 
        {
            
             if (strpos($_POST['message'], 'https://') !== false)
             {
                $msg_danger =  "Invalid Data Intered .The URL can't include in a query parameter.";
             }
             else
             {
                 if (preg_match('/^\d{10}$/',  $_POST['contact'])) 
                    {
                      $_POST['scheduled_date']    = htmlspecialchars($_POST['scheduled_date'], true);
                      $_POST['name']              = htmlspecialchars($_POST['name'], true);
                      $_POST['email']             = htmlspecialchars($_POST['email'], true);
                      $_POST['contact']           = htmlspecialchars($_POST['contact'], true);
                      $_POST['message']           = htmlspecialchars($_POST['message'], true);
          
                        unset($_POST['code']);
                        $id = $Uni->insert('enquiry',$_POST);
                        if ($id)
                        {
                          $msg_sussess = 'Request Send';
                        }
                    }
                    else
                    {
                        $msg_danger = "Invalid contact number. It must be exactly 10 digits and only contain numbers.";
                    }
             }
        }
  }
  else
  {
    $msg_danger = 'Invalid Captcha !';
  } 
}
?>

And HTTML Code

                <form action="" method="post" class="col-6 p-3">

                    <div class="row bg-light rounded">
                        <div class="col-12 py-4 text-center bg-dark text-white rounded">
                            <h3>Get In Touch</h3>
                            <p class="lead">We’re here to answer your questions and help you get started.</p>
                        </div>

                        <div class="col-12 py-3">
                            <?php include('Class/error.php'); ?>
                        </div>

                        <div class="col-md col-sm-12 form-group mb-3" data-for="name">
                            <input type="text" name="name" placeholder="Name" class="form-control" value="<?= ( isset($_POST['name']) ) ? $_POST['name']  : ''; ?>" required="">
                        </div>

                        <div class="col-md col-sm-12 form-group mb-3" data-for="email">
                            <input type="email" name="email" placeholder="E-mail" class="form-control" value="<?= ( isset($_POST['email']) ) ? $_POST['email']  : ''; ?>" required="">
                        </div>

                        <div class="col-md-12 col-sm-12 form-group mb-3" data-for="email">
                            <input type="number" name="contact" placeholder="contact " class="form-control" value="<?= ( isset($_POST['contact']) ) ? $_POST['contact']  : ''; ?>" required="">
                        </div>

                        <div class="col-12 form-group mb-3" data-for="textarea">
                            <textarea name="message" placeholder="Message" class="form-control" required="" rows="10"></textarea>
                        </div>

                        <div class="col-sm-3 mb-4 align-self-center text-start">
                            <img src="<?=BASE_URL;?>Class/captcha.php" class="img-fluid rounded" id="captcha">
                        </div>

                        <div class="col-sm-3 mb-4 align-self-center text-start">
                            <a href="javascript:void(0);" class="btn btn-success btn-sm" id="reload">Refresh </a>
                        </div>    

                        <div class="col-sm-6 mb-4 align-self-center">
                            <input type="text" name="code" class="form-control w-100" placeholder="Enter Chaptcha Code" required="">
                        </div> 

                        <div class="col-lg-12 col-md-12 col-sm-12 text-center">
                            <button type="submit" class="btn btn-primary btn-sm w-100" name="contact_us">Send <i class="bi bi-send-fill"></i></button>
                        </div>
                    </div>

                </form>