logged, whos logged, cant log

hows it going.

how do you set up that if a user is logged in noone else can log in using the same login details. i was thinking of putting a 0/1 filed in the database and
then if logged in you cant login again.

any one any sample codes to view whos logged in for an admin page

how would you write this statement
if username already exsists, please choose a different user name,
username chosen at registration shall be my primary key

: ;)

Set a bit (1 or 0) in the database for this. In the application's Session end event, you can reset the bit to 0 in the database.