There are several ways to assure or at least increase the chances that the user is coming in the way you expect them to:
To find the method they are using:
$REQUEST_METHOD or
$HTTP_ENV_VARS["REQUEST_METHOD"]
to find out if they came from your page:
HTTP_ENV_VARS["HTTP_REFERER"]
make sure this page is the one you submitted the form data from.
To grab your variables:
Either use the standard $varname, or use
$HTTP_POST_VARS['varname']
You can also include some sort of hidden form field that your script checks for when the data is POSTed. All these things together should give you a pretty good clue that the user came from where you expected them to.
Dave