foreach radio buttons

[Anon]

I finally got this to work for validating text boxes on a form:

foreach ($HTTP_POST_VARS as $name => $value)
{
$val = Check_Null_Field($value);
.
. // output error msg if NULL
.
}

Question is, how do I also get this to work for validating radio (option) buttons? I tried:

foreach ($HTTP_POST_VARS as $radio => $value)

and even:

foreach ($HTTP_POST_VARS as $type => $value)

but this does not work. The radio buttons are skipped as if they did not exist on the form.