I want to offer a guestbook application on my website for all of my members.Each member has his own folder and his own guestbook files.
Now about session that is created for each user.
 I want to know if other users can access other users' admin when they login to their account while the session name is the same for all redirect pages.can this cause problems.?

No problems there. Each user who visits your site share independant sessions.

IMO Each user should have a primary key field in the user BD table (preferably an auto incrimenting integer) this field is unique to each user. When they login set your session("someName") = that value. Now each user has this session variable set upon login however they are populated with unique values

Wind is your friend
Matt