You use $POST_ when you are grabbing information from a form (person fills out a form - say a name - and you want to use that information on the next page) and you use $GET_ when you are you are getting the variable from the url (url/page.php$do=variable). So, try using $GET_['do'] or $REQUEST_['do'].
(I think I got that right, I do need more coffee this morning!)
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