The answer would be:
Suppose your URI is "querystring1.php?key=value" then
$_REQUEST['key']
would return "value", or whatever is passed as a query parameter value.
If you don't want to leverage this standard "key=value" form of passing query parameters, you have to look at the URI of the HTTP request itself and interpret it the way you wish. In particular, the whole query string can be accessed as
$_SERVER['QUERY_STRING']
. For related detail of URI please see, for example:
http://www.phpf1.com/tutorial/get-current-page-url.html[
^].
But it would not make much sense — please see my comment to the question. By the way, one widely used method of passing data via a "POST" HTTP request which is not supposed to be modified by the user is using a hidden control with the value you want to pass. The value is passed the same way you passed, for example, "txt1" value.
—SA