TP框架的 redirect方法是怎么实现跳转的?一看源码，完全不能理解啊

<br /><br />    function redirect($url, $time=0, $msg='') {<br />        //多行URL地址支持<br />        $url = str_replace(array("\n", "\r"), '', $url);<br />        if (empty($msg))<br />            $msg = "系统将在{$time}秒之后自动跳转到{$url}！";<br />        if (!headers_sent()) {<br />            // redirect<br />            if (0 === $time) {<br />                header('Location: ' . $url);<br />            } else {<br />                header("refresh:{$time};url={$url}");<br />                echo($msg);<br />            }<br />            exit();<br />        } else {<br />            $str = "<meta http-equiv='Refresh' content='{$time};URL={$url}'>";<br />            if ($time != 0)<br />                $str .= $msg;<br />            exit($str);<br />        }<br />    }<br />