PHP错误:在没有任何html/cookie/空间的情况下，无法修改标头信息

我的脚本有一个奇怪的错误，如果URL以"/"结尾，我只想将用户重定向到同一个URL，而结尾没有任何"/"。

我得到这个错误：

Warning: Cannot modify header information - headers already sent by (output started at /home/httpd/vhosts/xxx/httpdocs/series.php:1) in /home/httpd/vhosts/xxx/httpdocs/series.php on line 7
Warning: Cannot modify header information - headers already sent by (output started at /home/httpd/vhosts/mxxx/httpdocs/series.php:1) in /home/httpd/vhosts/xxx/httpdocs/series.php on line 8

代码：

<?php
$urlLast = $_SERVER['REQUEST_URI'];
$urlLast = substr($urlLast, -1);
if (($urlLast == '/') && (!strstr($_SERVER['REQUEST_URI'], 'en-streaming')))
{
    $newURL = substr($_SERVER['REQUEST_URI'],0,-1).'-en-streaming';
    header("Status: 301 Moved Permanently", false, 301);
    header("Location: ".$newURL."");
}