如何显示进度/加载使用PHP将数据添加到MYSQL

I am creating a user registration form and i want to show a loading/progress on my page as soon user click submit button. Here Is my code please suggest me something to achieve this. here below is process.php file

 if(isset($username)){
    $con = new dbConnection();
    $con->connect();
    $con->add_user($username,$email,$password,$contact);
    $db_class = 'ui success message';
    $db_message_header = 'header';
    $db_header_message = 'Your user registration was successful.';
    $db_msg= 'You may now log-in with the username you have chosen';
    $form_load = 'loading'; //Class for adding loading class
    sleep(4);
   header('location:index2.php');

}

Here is index.php

  <form class="ui form attached fluid segment <?php echo $form_load; ?>" action="<?php echo $_SERVER['PHP_SELF']; ?>" method="POST" id="form">
        <div class="field">
          <label>Username <span id="err">*</span></label>
          <input placeholder="Username Name" type="text" name="username" class="ui input success" value="<?php if(isset($username)){echo $username;} ?>" required>
        </div>
          <input type="submit" name="submit">Submit</input>
     </form>