Session variables lost

Hello, I am using session variables using the format of $_SESSION["XXX"]. Sometimes, some of the session variables are lost when trying to access a different page. The list is consistent. At the end of the current page, I display the session variable value and it's working but on the start of the next page it's gone. I've seen some information about it saying to add page_start() and page_end() using phplib. I do not know where to find phplib and I prefer not to use plain php. Any idea, someone? Regards, Reuven