<?php
$user_id = 0;
$table_prefix = 'elgg_'; // get from engine/settings.php
if (! empty($_COOKIE['Elgg'])) {
    $session_id = mysql_real_escape_string($_COOKIE['Elgg']);
    $sql = ("SELECT `data` FROM `{$table_prefix}users_sessions` WHERE `session` = '{$session_id}'");
    $result = mysql_query($sql);
    if ($row = mysql_fetch_array($result)) {
        $data = explode('attributes";', $row[0]);
        if (!empty($data)) {
            $session = unserialize($data[1]);
            $user_id = $session['guid'];
        }
    }
}