substr is inside the post variable name:
$_POST['substr($_POST["zipcode"],0,3)']
using the simple recordset interface, you cannot use functions like this, yu can only enter the variable name.
set it to use a URL variable named zipcode.
then add code before the recordset to set the URL variable:
<?php if(isset($_POST["zipcode"])) $_GET['zipcode'] = substr($_POST["zipcode"],0,3); ?>