PHP cannot obtain Referer troubleshooting, referer troubleshooting_PHP tutorial

<?php

date_default_timezone_set(<span class="str">'Asia/Shanghai'</span>);<span class="rem">//设置默认时区</span>
header(<span class="str">'content-type:text/html; charset=utf8'</span>);<span class="rem">//设置返回头信息</span>

echo date(<span class="str">'Y-m-d H:i:s'</span>);
echo <span class="str">"<br/>\r\n"</span>;

echo <span class="str">'PHP获取Refer是：'</span>;
$refer=isset($_SERVER[<span class="str">'HTTP_REFERER'</span>])?$_SERVER[<span class="str">'HTTP_REFERER'</span>]:<span class="str">'No refer found'</span>;
echo $refer;
echo <span class="str">"<br/>\r\n"</span>;

echo <span class="str">'JS获取Refer是：'</span>;
    echo <span class="str">"<script>document.writeln(document.referrer);</script>"</span>;
echo <span class="str">"<br/>\r\n"</span>;