Im sorry, I think you misunderstood.
The code is correct if u just want to replace it, like normal case:
$UpdateQuery->bindColumn("ProvningPlatserBokade", "i", "".($WADABokning_update->getColumnVal("BokningAntal") ."", "WA_DEFAULT");
But I want it to be added. As an amateur I think like: ("ProvningPlatserBokade", "i", "".++.($WADABokning_update->getColumnVal("BokningAntal") ."", "WA_DEFAULT");
There the ++ tell it to be added. But this doesn't work.