在php中提交表单后无法显示成功消息

Question

我正在尝试重定向到同一页面，并在php中提交表单后显示成功消息。当您提交表单时，用户被带到registration_detail，没有消息已发送该表单。我尝试了各种方法。我做不到请帮助我。

<?php 
   require_once "conn.php";
  // $reg_id1 = intval($_POST['reg_id']);
    $posted = false;

   $registration_date = $building_use_id = $building_category =$reg_id="";
   if($_SERVER["REQUEST_METHOD"] == "POST"){
      $posted = true;
    $reg_id=intval($_POST['reg_id']);
    $registration_date = trim($_POST["registration_date"]);
     $building_use_id = trim($_POST["building_use_id"]);
     $building_category = trim($_POST["building_category"]);
           $sql = "INSERT INTO bps_registration_application (registration_date, building_use_id, building_category,reg_id) VALUES (?, ?, ?, ?)";   
       if($stmt = mysqli_prepare($conn, $sql)){
        mysqli_stmt_bind_param($stmt, "ssss",$registration_date, $building_use_id, $building_category,$reg_id);
        $reg_id=intval($_POST['reg_id']);
        $registration_date=$registration_date;
        $building_use_id = $building_use_id;
        $building_category = $building_category;
          if(mysqli_stmt_execute($stmt)){
   if(!empty($reg_id)){
       $success = "Message successfully sent";
       echo "<script type='text/javascript'>alert('submitted successfully!')</script>";
         header("location: registration_detail.php?id=".$reg_id);
           exit();
   }
   else
   {
     header("location: registration_detail.php");
           exit();   
   }

         }
         else{
           echo "Something went wrong. Please try again later.";
         }
      }
    echo $success;
   mysqli_close($conn);
   }

   ?>