For online users it's _ShardCurrentUser from TB_User but it's getting updated every 1 min
Hint:
PHP Code:
$query=mssql_query("SELECT top 1 * FROM _ShardCurrentUser WHERE nShardID = 64 ORDER BY nID desc");
$row=mssql_fetch_array($query);
Level exp : SRO_VT_SHARD=>_RefLevel