basically you have a variable with the same name as your session. ex:
$_SESSION['var1'] = null;
$var1 = 'something';
which will reproduce this error. you can stop PHP from trying to find existing variables and warning you about them by adding these lines to your script:
ini_set('session.bug_compat_warn', 0);
ini_set('session.bug_compat_42', 0);
these values can be set in php.ini or .htaccess as well