Is User-Agent consistent?

<?php

session_start
();

if (isset(
$_SESSION['HTTP_USER_AGENT']))
{
    if (
$_SESSION['HTTP_USER_AGENT'] !=
    
md5($_SERVER['HTTP_USER_AGENT']))
    {
        
/* Prompt for password */
        
exit;
    }
}
else
{
    
$_SESSION['HTTP_USER_AGENT'] =
    
md5($_SERVER['HTTP_USER_AGENT']);
}

?>