session side-effect which existed until PHP...

When I launch this code, I get "Warning: Unknown(): Your script possibly
relies on a session side-effect which existed until PHP 4.2.3. Please be
advised that the session extension does not consider global variables as
a source of data, unless register_globals is enabled. You can disable
this functionality and this warning by setting session.bug_compat_42 or
session.bug_compat_warn to off, respectively. in Unknown on line 0"

Since it's not my server, how do I modify this to avoid the complaint?

$variable = $_SESSION['plaintext'];

if ($variable != bob){

echo "do stuff";