// add a default 'sc' value
$options['sc'] = 'sc';

// grab values of App_Code field from Modules table
$mods = sc_query('SELECT App_Code FROM Modules',MYSQL_ASSOC,1);

// loop thru and set each one as an option
if (is_array($mods)) {
   foreach($mods as $mod) {
       $options[$mod['App_Code']] = $mod['App_Code'];
   }
}
