basically what ya want is an error page to popup? or do ya just want the eror to show up? cause IF it was me.. I would create a script that at has a
if(!$do == "ran"){
do this
}
if ($do == "ran"){
do this
}
then in the form put a hidden field called $do and then use a bunch of $_POST calls to get the data which if was never ran would be null and if ran would be something.. then test for the something to see if they entered all the somethings you wanted them to and if not it would still keep the values they did but wouldn't process the rest of the script until they entered it all
