To define options of combo you should use ComboConnector on the server-side and specify the connector file in the loadXML(url) method on client-side:

client-side:

var combo=new dhtmlXCombo("combo_zone2","alfa2",200);
combo.loadXML("connector.php");

server-side:

<?php 
require_once("../codebase/connector/combo_connector.php");
$res=mysql_connect("localhost","root","");
mysql_select_db("mydb");
 
$data = new ComboConnector($res, "MySQL");
$data->render_table("categories","id","valueColumn, labelColumn");
?>

• Names of the fields can have aliases (value or label) to identify the appropriate attribute.
  
  

  $data->render_sql("SELECT *, CONCAT(FirstName, LastName) as label FROM table1","id","id,FirstName(label)");

  Note, in the filtering mode a combo filters data by the “label” field.