无法修改标头输出已启动警告

Question

所以我收到这个错误：

警告：无法修改标头信息 - 已在第55行的/Applications/MAMP/htdocs/add_user.php中发送的标头（在/Applications/MAMP/htdocs/add_user.php:50处开始输出）

第55行的代码是：

55 header(sprintf("Location: %s", $insertGoTo));

脚本开始于：

 <?php require_once('Connections/connHotel.php'); ?>
 <?php
 function GetSQLValueString($theValue, $theType, $theDefinedValue = "", $theNotDefinedValue = "") 

  $theValue = (!get_magic_quotes_gpc()) ? addslashes($theValue) : $theValue;

  switch ($theType) 
    case "text":
      $theValue = ($theValue != "") ? "'" . $theValue . "'" : "NULL";
      break;    
    case "long":
    case "int":
      $theValue = ($theValue != "") ? intval($theValue) : "NULL";
      break;
    case "double":
      $theValue = ($theValue != "") ? "'" . doubleval($theValue) . "'" : "NULL";
      break;
    case "date":
      $theValue = ($theValue != "") ? "'" . $theValue . "'" : "NULL";
      break;
    case "defined":
      $theValue = ($theValue != "") ? $theDefinedValue : $theNotDefinedValue;
      break;
  
  return $theValue;


$editFormAction = $_SERVER['PHP_SELF'];
if (isset($_SERVER['QUERY_STRING'])) 
  $editFormAction .= "?" . htmlentities($_SERVER['QUERY_STRING']);


if ((isset($_POST["MM_insert"])) && ($_POST["MM_insert"] == "userform")) 
  $insertSQL = sprintf("INSERT INTO clients (title, firstName, lastName, address1, address2, town, province, country, postCode, telephone, email) VALUES (%s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s)",
                       GetSQLValueString($_POST['title'], "text"),
                       GetSQLValueString($_POST['firstName'], "text"),
                       GetSQLValueString($_POST['lastName'], "text"),
                       GetSQLValueString($_POST['address1'], "text"),
                       GetSQLValueString($_POST['address2'], "text"),
                       GetSQLValueString($_POST['town'], "text"),
                       GetSQLValueString($_POST['province'], "text"),
                       GetSQLValueString($_POST['country'], "text"),
                       GetSQLValueString($_POST['postcode'], "text"),
                       GetSQLValueString($_POST['telephone'], "text"),
                       GetSQLValueString($_POST['email'], "text"));

mysqli_select_db($connHotel, $database_connHotel);
  $Result1 = mysqli_query($connHotel, $insertSQL) or die(mysqli_error($connHotel));

  $insertGoTo = "booking_details.php?email=$email";
  if (isset($_SERVER['QUERY_STRING'])) 
    $insertGoTo .= (strpos($insertGoTo, '?')) ? "&" : "?";
    $insertGoTo .= $_SERVER['QUERY_STRING'];

我不明白警告的含义。