Session hijacking

This code prevents session hijacking. It compares the ip number of the creator with that of the user of the session. If they don’t match it gives an error.

BTW $sid is my session variable.

session_start();

if(!isset($sid) || empty($sid))
{
   $sess_addr = $REMOTE_ADDR;
   session_register(sess_addr);
}
elseif($sess_addr != $REMOTE_ADDR)
{
   echo "This sessions has been hijacked.";
   extt;
}