ช่วยดูโคดให้หน่อยครับ.. Fatal error: Call to undefined function session_is_registered()

<?
session_start();
?>
<?
if (empty($status)) $status=NULL;
if ($status!="logout")
{
session_start();
require_once("../include/check_data.php");
if(!session_is_registered("valid_user"))
{
require_once("../include/header.php");
header_html(by"Home");
}
else
{
require_once("../include/header1.php");
header_html1("Home");
require_once("../include/check_data.php");
$hwel = check_valid_user();
}
}
else
{
require_once("../include/header.php");
header_html("Home");
}
if (empty($hwel)) $hwel = NULL;
?a
<table width="850" valign="top" border="0" cellpadding = "0">
<tr valign="top" >aa
<td width="130" bgcolor="#6699CC">
<?
require_once("../chapter19/left_menu.php");
?>
</td>
<td width="700">
<?
require_once("../chapter19/index.php");
?>
</td>
</tr>
</table>
<?
require_once("../include/footer.php");
?>

<?php
<?
session_start();
?>
<?
if (empty($status)) $status=NULL;
if ($status!="logout")
{
session_start();
?>

<?php
if(!isset($_SESSION["valid_user"])) แบบนี้แทน session_is_registered
?>

<?
require_once("../include/function.php");
require_once("../include/header.php");
require_once("../include/check_data.php");
require_once("../include/connect_db.php");
session_start();
if(!filled_out($HTTP_POST_VARS))
{
	echo "?????????????????????????????????? ?????????????????";
	header_html("???????????????????????????");
	require_once("../include/footer.php");
	exit;
}
//email ??????????
if(!valid_email($email))
{
	header_html("???????????????????");
	echo "???????????????? ??????????????????????????";
	require_once("../include/footer.php");
	exit;
}
//?????????????????
if($password1 != $password2)
{
	header_html("???????????????????");
	echo "????????????????? ??????????????????????????";
	require_once("../include/footer.php");
	exit;
}
//?????????????????????????
if(strlen($password1)<6 || strlen($password2)>16)
{
	header_html("???????????????????");
	echo "??????????????????????????????? 6 ??? 16 ??????????????????????????";
	require_once("../include/footer.php");
	exit;
}
//?????????
$reg_result = register($username,$password1,$answer,$email);
if($reg_result)
{
	$valid_user = $username;
	session_register("valid_user");
	header_html("?????????????????????");
	require_once("register_show.php");
}
else
{
	header_html("???????????????????");
	echo $reg_result;
	require_once("../include/footer.php");
	exit;
}
require_once("../include/footer.php");
?>


อีกไฟล์ check_data




<?
function check_valid_user()
{
global $valid_user;
if(session_is_registered("valid_user"))
		$str = "<tr><td colspan=\"3\"><font face = \"MS Sans Serif\" color=\"#000000\" size=\"3\"> ??????????????? $valid_user</font></td></tr>";
else
		$str = "<center>????????????????</center><br>";
		return $str;
}
//=============================================================================
function filled_out($form_vars)
{
	foreach($form_vars as $key=>$value);
	{
		if(!isset($key)||($value==""))
			return false;
	}
	return true;
}
//==========================================================================
function check_admin()
{
  global $authuser;
  if (session_is_registered("authuser"))
  {
      echo "<tr><td><center>????????????????????? $authuser</center>";
      echo "</td></tr>";
	  return true;
  }
  else
  {
     echo "<tr><td><center>???????????????? $authuser</center></td></tr>";
	 return false;
     exit;
  }  
}
//========================================================================
function valid_email($add)
{
//?????????????
if(ereg("^[a-zA-Z0-9_]+@[a-zA-Z0-9\-]+\.[a-zA-Z0-9\-\.]",$add))
	return true;
else
	return false;
}

?>

<?php
function filled_out($form_vars)
{
foreach($form_vars as $key=>$value);
{
if(!isset($key)||($value==""))
return false;
}
return true;
}
?>

<?php
function filled_out($form_vars)
{
if(!is_array(form_vars) or count(form_vars) == 0) return false; // เพิ่มนี่เข้าไปอาจช่วยได้
foreach($form_vars as $key=>$value);
{
if(!isset($key)||($value==""))
return false;
}
return true;
}
?>

<?
require_once("../include/function.php");
require_once("../include/header.php");
require_once("../include/check_data.php");
require_once("../include/connect_db.php");
session_start();
if(!filled_out($HTTP_POST_VARS))
{
echo "เกิดความผิดพลาดในการป้อนรายละเอียด";
header_html("การลงทะเบียนเกิดความผิดพลาด");
require_once("../include/footer.php");
exit;
}
//email ไม่ถูกต้อง
if(!valid_email($email))
{
header_html("เกิดความผิดพลาดขึ้น");
echo "อีเมลไม่ถูกต้อง กรุณากลับไปตรวจสอบอีกครั้ง";
require_once("../include/footer.php");
exit;
}
//รหัสผ่านไม่ตรงกัน
if($password1 != $password2)
{
header_html("เกิดความผิดพลาด");
echo "รหัสผ่านไม่ตรงกัน กรุณากลับไปตรวจสอบอีกครั้ง";
require_once("../include/footer.php");
exit;
}
//ตรวจสอบความยาวของรหัส
if(strlen($password1)<6 || strlen($password2)>16)
{
header_html("เกิดความผิดพลาด");
echo "ความยาวของรหัสไม่อยู่ในช่วง 6 ถึง 16 กรุณากลับไปตรวจสอบอีกครั้ง";
require_once("../include/footer.php");
exit;
}
//ลงทะเบียน
$reg_result = register($username,$password1,$answer,$email);
if($reg_result)
{
$valid_user = $username;
session_register("valid_user");
header_html("ลงทะเบียนเรียบร้อย");
require_once("register_show.php");
}
else
{
header_html("เกิดความผิดพลาด");
echo $reg_result;
require_once("../include/footer.php");
exit;
}
require_once("../include/footer.php");
?>

<?php
if(!isset($_SESSION["valid_user"])) แบบนี้แทน session_is_registered
?>