Fatal error: Call to undefined function: session_start()

[da2]

Este es inicio del codigo de la pagina index.php

Código:

<?
session_start();
define("IN_INDEX",1);
error_reporting  (E_ERROR | E_WARNING | E_PARSE);
include "database.php";
include "config.php";
include "function.php";
include "pclzip.lib.php";
include "lang.php";
$USER=$SET=array();
$db=new database($sql_host,$sql_user,$sql_pass,$sql_database,"");
$input=parse_incoming();

checkIP();
checkSess();
loadUser();

//encode query string
$param=encryptStr(base64_encode("&uid=$USER[uid]&guest_id=".getCookies('guest_id')));
$filetype=str_replace(',,',',',$USER[filetype]);
if(substr($filetype,strlen($filetype)-1,1)==',')
{
    $filetype=substr($filetype,0,strlen($filetype)-1);
}
$sizelimit=$USER[sizelimit];
$baseUrl='index.php?';
//log out
if($input[logout]==1)
{
    $_SESSION[logined]=0;
    header("location:index.php");
}

Agradecería que alguien me saque del apuro.