I helped someone create a online study. The survey consists of different surveys and games strung together. At the beginning of the study, I inserted the person's info into a participant database and assigned him a unique serial number. For each other survey/game, I passed the serial along in a php session variable and inserted it into each survey/game database table along with the other information.
However, in some cases the session variable is lost and is NULL, which messes things up. What types of things can cause this? Would refreshing the page clear session variables? Is there any way for me to ensure that the user doesn't lose a session variable?