validating listbox

[tjbarr]

Please choose the Variables you want for Source -
U.S. Bureau of Labor Statistics 2 digit Employment:

listbox of Variables Listbox of years

If the user picks a variable but doesn't pick at least one year I would like an error message " must pick a year for the variable to appear either before or after the submit. Help

[tjbarr]

i know i need an isset :
e.g.

if isset (listvariable) and ! isset (listyr)
{
echo " Must pick a year for variable choice"
}

Am I on the right track, and if so how would I do this validation on submit?

[sidney]

if(isset($_POST['submit']))
{
if (isset (listvariable) and ! isset (listyr) )
{
echo " Must pick a year for variable choice"
}
else
{
//dowhat ever you want as vars are set and form has been submitted
}

}