php获取发送给用户的header信息的方法

<?php
  header("Expires: Sat, 12 Dec 1989 05:30:00 GMT");
  echo "This is some output.<br />";
  echo "Headers sent are:<br /> <UL>";
  $headers = headers_list();
  foreach($headers as $header) {
    echo "<LI>$header</LI>";
  }
  echo "</UL>";
?>