Knowing when a visitor leaves.

Hi all I have a small web shop, what happens in it is that when a visitor clicks a displayed object ( adds it to his shopping cart) then it is marked as booked in the MySQL table, the problem is that when the visitor leaves without checking out, i.e. just chooses sumthing and decides to leave the object in the table remains booked and is not displayed to other visitors, is there any way that i can see when a person leaves the webshop, when he closes the browser window, the variables his session is destroyed automaitically right ? but that does'nt change allow me to attach a subroutine so that the changes are made in the tables. the solution i have thought of is to execute a subroutine when the session expires, but how do i do that, how do i set the time for session to expire and how do i attach a function/subroutine to that. the final question is that on this list when we use the word "client" it means the person hu is visiting the website right as in "client browser", and how does the SSL "client " differ from this. thanx alot in advance -- Saquib Farooq @ Systems @ SDNPK Islamabad