Newsconomy

When it renames the session id it does not remove the old session, leaving it active and potentially usable by a would be hacker.

Free online PHP book.

It is all well and good to see the line where your code went wrong, but how did it get to that line? Debugging complex scripts can sometimes be a nightmare because objects call functions, which call other objects and other functions, and so on - you end up with a nest of calls that make tracing the problem difficult. To make your life easier, you can use the function debug_backtrace().