Export query to csv with php

I have the following code to export a query to csv with php but it does not work, you know how I can get export the query to csv with php

 require '../conn.php';

  $sql = '(SELECT *) 
  UNION ALL
  (SELECT * FROM senders INTO OUTFILE 'prueba.csv' FIELDS TERMINATED BY ',' ENCLOSED BY '"' LINES TERMINATED BY '\n');'; 
  $result = mysqli_query($conn, $sql);
  if (mysqli_num_rows($result) > 0) {
    $response->state = true;
    echo json_encode($response);
  } else {
    $response->state = false;
    echo json_encode($response);
  }

$ response I have declared it in "conn.php"