You can use this function to return the unique Steam user
id of the user currently logged into the Steam client. This is
a unique 64 bit value that can be used to identify individual users
and not their on screen user name (this can be found using
the function steam_get_persona_name).
global.u_id = steam_get_user_steam_id();
The above code will set a global variable to the current users unique Steam ID if the Steam client API is correctly initialised.