[thetwistedpanda]

There are dozens of solutions to this little problem, but the easiest way (while not the correct way) is to force a specific universe of Steam IDs regardless of what the client is using. That is to say, if the client is using the STEAM_1 branch of identifications, simply force the 6th character of the string to a 0, so that they're identified as the STEAM_0 branch.
Without looking at the source code, here's what I mean:

PHP Code:

decl String:sSteam[32];
GetClientAuthString(client, sSteam, sizeof(sSteam));
sSteam[6] = '0';
//Poof! STEAM_1 becomes STEAM_0