Is a list box those multiple selects that you can hold ctrl down and select as many as you want? If so, you gotta make sure the name for the select is an array, then you can grab all the values
<select name="bla[]" multiple>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>
The [] after the name will put the values into an array called $bla
Now to get the values, you could do it a few different ways, I guess it all depends what you're doing. If you just want to echo them, you could use a foreach loop
foreach($_POST['bla'] AS $value) {
echo $value . "<br>";
}
You could obviously modify the foreach loop to do something different than echoing the values they selected.
Cgraz