I have a stock that may help you:
PHP Code:
stock GetSteamID(client, String:authstr[], maxlen)
{
GetClientAuthString(client, authstr, maxlen);
if(StrEqual(authstr, "STEAM_ID_LAN", false))
GetClientIP(client, authstr, maxlen);
ReplaceString(authstr, maxlen, "STEAM_0:", "", false);
ReplaceString(authstr, maxlen, "STEAM_1:", "", false);
}
Use it like this:
PHP Code:
decl String:buffer[256];
GetSteamID(client, buffer, sizeof(buffer));
// buffer now holds the steam ID