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Mano sprendimas butu toks:

Kazkoki tai fielda duombazej tureti prie user'io. pvz: last_time_viewed_forum (TIMESTAMP)
Jeigu zmogus yra prisijunges (berots iMEMBER budavo tikrinimas ar online) - Gauni $userdata['last_time_viewed_forum'].
Ir tada sakykim taip:
https://hastebin.com/baxitolafi.xml

Dabar kur lenkiu... Sitas kodas, naudotu ajax, bet manau, jog su ajax tau dar toli kol kas.
Tad galima daryti sitaip:
https://hastebin.com/etaqagoniy.xml

Tada, sukursi metoda tam paciam faile:
https://hastebin.com/cajudehowi.xml