会话被忽略了

Question

我试图在PHP中创建一个基于会话的小型语言切换脚本。由于某种原因会话条件没有通过，就好像我尝试检查的每个会话都已经设置好了。我错过了什么吗？

<?php
    session_start();
        echo 'session started';
        if ( !isset($_SESSION[ 'language' ]) && empty($_SESSION[ 'language' ]) ) {
          echo 'set new session'; // Never reach this point, it seems the browser already has the session in store. Tried changing the session name several times.
          redirect();
          $_SESSION[ 'language' ] = true;
        }
        function redirect() {
          echo 'running redirect()';
          $language = explode( ',', $_SERVER['HTTP_ACCEPT_LANGUAGE'] );
          switch( $language[0] ) {
            case 'de-DE':
            case 'sv-SE':
            case 'no-NO':
            case 'dk-DK':
              header( 'Location: http://sample.com/' . substr($lang, 0, -3) . '/' );
            break;
          }
        }
?>