I have a number of clients sites running osCommerce or a derivative of osCommerce. As I deal with the PHP5 migration, I will post here problems/ solutions that I encounter so that others can successfully make the migration as well.
Problem 1: Global variables no longer supported
Note: there are solutions out there to deal with this, but most of these "solutions" aren't any better than simply re-enabling global variables within an .htaccess file. From a security perspective, any security risks associated with global variables have already most likely been addressed. So, here's the lines to add to your .htaccess file for osCommerce:
php_value register_globals 1
php_value register_long_arrays 1
Problem 2: Another issue comes up in your admin/includes/classes/upload.php file that will result in the administrative login screen not loading. If you view your PHP error logs, you will see an entry like Cannot re-assign $this in /big/dom/xdomain/.../admin/includes/classes/upload.php. This is easy enough to fix, just open admin/includes/classes/upload.php and on about line 30, comment out the following line:
so that it reads:
//$this = null;
If you are running osCommerce, or a derivative thereof, and experience problems after implementing these fixes, feel free to post here.