Oh, that's what you're talking about. Well, I have the answer.
Your 'register_globals' php.ini setting is set to off. You probably had it on before. When it's on, the $myvar variable will get created for you automatically by PHP. Since it's now off, PHP will not create it.
It is best to keep it off for security reasons and program accordingly. This means using $_GET in this instance. Example:
$myvar = isSet($GET['myvar']) ? $GET['myvar'] : ''; // Set to a default you want
if ('this' == $myvar) {
...
If you want it to behave the way it does with register_globals being on, but without actually turning it on, then you can use import_request_variables() or extract():
http://us2.php.net/manual/en/function.import-request-variables.php
🙂
EDIT: Posted too late.