HATA: Fatal error: Call to undefined function session_is_registered()

if(session_is_registered("randomuserid") and session_is_registered("adminID")){ #session if

<?php
ob_start();
session_start();
include "system/db_baglan.php";
echo "<LINK href='file/css/defter.css' type='text/css' rel='stylesheet'>";
if(session_is_registered("randomuserid") and session_is_registered("adminID")){ #session if
header("location:administrator.php");
}else{
echo "<form action='login.php' method='post'>
<table border='0' width='100%' id='table1' cellpadding='0' style='border-collapse: collapse' height='100%'><tr><td align='center'>
	<table border='0' width='400' id='table2' cellpadding='0' style='border-collapse: collapse'><tr><td height='40' background='tema/$renk/1.gif'>
			<p align='center'><b><font size='2' color='#FFFFFF'>Yönetici Girişi</font></b></td></tr><tr><td height='20' background='tema/$renk/2.gif'>&nbsp;</td></tr><tr><td>
			<table border='0' width='100%' id='table3' cellpadding='0' style='border-collapse: collapse'>
				<tr>
					<td width='151' height='100' rowspan='7'>
					<p align='center'><a href='index.php' title='Defter Ana Sayfa'>
					<img border='0' src='images/icon/guvenlik.gif' width='50' height='56'></a></td>
					<td width='83' height='15'></td>
					<td height='15'></td>
				</tr>
				<tr>
					<td width='83' height='20'>Kullanıcı Adı</td>
					<td height='20'>
					<input type='password' name='kulladi' class='text' maxlength='32' style='width: 100; height: 18'></td>
				</tr>
				<tr>
					<td width='83' height='5'></td>
					<td height='5'></td>
				</tr>
				<tr>
					<td width='83' height='20'>Şifre</td>
					<td height='20'>
					<input type='password' name='sifre' class='text' maxlength='32' style='width: 100; height: 18'></td>
				</tr>
				<tr>
					<td width='83' height='5'></td>
					<td height='5'></td>
				</tr>
				<tr>
					<td width='83' height='20'>&nbsp;</td>
					<td height='20'><input type='submit' name='giris' class='giris' value='' style='width: 69; height: 20'></td>
				</tr>
				<tr>
					<td width='83' height='15'></td>
					<td height='15'></td>
				</tr>
			</table>
			</td></tr><tr><td height='20' background='tema/$renk/3.gif'>&nbsp;</td></tr><tr><td height='20' background='tema/$renk/1.gif'>&nbsp;</td></tr></table></td></tr></table></form>";
}
ob_end_flush();
?>