I think it's related to older versions of PHP. If you can upgrade to the latest 5.3 version, I would. If only for security reasons.
To get around the error without upgrading PHP, and without using the ini_set directive in every PHP file, you can set the options in a custom php.ini, or in your .htaccess, depending on how PHP is set up on your system.
If you can override the default php.ini on your system, create a file php.ini and place it in your root web document folder (IE: public_html, www, or htdocs) then add the following two lines to the file:
session.bug_compat_warn = 0
session.bug_compat_42 = 0
You may have to restart Apache, and/or add the path to your php.ini file ... again it depends on your server setup.
If you don't have access to a custom php.ini file, you can add the following two lines to your .htaccess file in your root web documents folder:
php_flag session.bug_compat_42 0
php_flag session.bug_compat_warn 0
Those two options can disable the warning globally, but the best solution is probably either to upgrade PHP or change the way you name your variables.