how can i export button to show csv file in php

How can i show to export button in csv file. Download Simple Export CSV file in PHP .

Problem : file can not show only data printed on page. What’s the problem with the code? i can not understand.enter image description here

 function car_data_callback()
// {

//  //Database connections
//  $database_server     = ""; 
//  $database_username = ""; 
//  $database_password = ""; 
//  $database_name     = ""; 
 
//  // Create database connection 
//      $connection = mysqli_connect($database_server, $database_username, $database_password, $database_name);

//  // Check connection
//      if($connection === false)
//      {
//          die("ERROR: Could not connect. " . mysqli_connect_error());
//      }

//      $query = "SELECT * FROM wpsn_car_price_table";
//      $result = mysqli_query($connection, $query);
//      $number_of_fields = mysqli_num_fields($result);
//      $headers = array();
        
//      for ($i = 0; $i < $number_of_fields; $i++) 
//      {
//          $headers[] = mysqli_field_name($result , $i);
//      }

//      $fp = fopen('php://output', 'w');
//      if ($fp && $result) {
        
//          header('Content-Type: text/csv');
//          header('Content-Disposition: attachment; filename="export.csv"');
//          header('Pragma: no-cache');
//          header('Expires: 0');
//          fputcsv($fp, $headers);

//          while ($row = $result->fetch_array(MYSQLI_NUM)) {
//              fputcsv($fp, array_values($row));       
        
//          }
//          die;
//      }

//  }

//      function mysqli_field_name($result, $field_offset)
//          {
//             $properties = mysqli_fetch_field_direct($result, $field_offset);
//              return is_object($properties) ? $properties->name : null;
//          }