function not working after header call

Hi, I just don't get it...I have a form where the user enters name, surname and a personal code. The action page checks for correct input which is only letters for the two names and only numbers for the code. If there is a wrong input in one of the fields a variable is set which is then (amongst others) passed in the URL to the page containing the form. formscript.php?code=1&name=1&surname=1&...... // for all three fields wrong, if they are ok they aren't passed Now in order to highlight the fields with worng input I worte a function. function wrong_input($var) global $var; echo isset($var) ? "<font face=\"Times\" size=\"2\" color=\"#FF0000\">" : "<font face=\"Times\" size=\"3\" color=\"#000000\">"; } This function is called in the <td> like this: <td><?php wrong_input($code); ?>Code</font></td> It is executed the first time but if the input is wrong and the user is sent back to the form page it is not. Desperately trying to get it to work I tried lots of combinations like using print in the function or use echo function($var) in the <td> but nothing worked out. I'm completely stuck. Any hlep is greately appreciated