Php -- what is this code doing

I'm particularly curious about the  line that starts   'return isset  ... '
Does a FORM need the data that this line would return in order to be Submitted?

Thanks

The PHP function isset returns true or false, so I am having trouble with the structure of this statement.. Within the isset parentheses is a true false statement using T/F shorthand called ternary operators. It says, if the parameter has been set, then return that value. If it isn't set, return the default value. So why is it in an isset statement? It should return true either way.
Have you tried echoing out what is returned?
It seems like this function is choosing a value for one element of the form, either the value the person entered or chose, or the form element is filled with the default value.
Someone correct me if I am wrong, maybe I am about to learn something new.

Within the isset parentheses is a true false statement using T/F shorthand called ternary operators.

False, the return on isset is what is used to evaluate the conditional. These 2 stuctures are identical:

return isset($HTTP_GET_VARS[$parameter_name]) ? CCStrip($HTTP_GET_VARS[$parameter_name]) : $default_value;

// Same as
if( isset($HTTP_GET_VARS[$parameter_name]) ) {
   return CCStrip($HTTP_GET_VARS[$parameter_name]);
} else {
   return $default_value;
}

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No points please, just trying to clarify.

Thanks Derokorian.