I was helping someone else with this issue. Actually the variables were being passed in the URL string in a non-www URL, and going to a URL with www, we thought that may be the problem. Just wondering if anyone here might be able to confirm if that is the case.
For example, going from http--domainDotcom/file.php?col=bl&img=default&c=4&i=1 and then go to http--wwwDotdomainDotcom/file.php returns all empty strings from session vars.