AJAX and PHP

Chapter 4}}{// display error messagedisplayError(e.toString());}}else{// display error messagedisplayError(xmlHttp.statusText);}// read server's responsefunction readResponse(){// retrieve the server's responsevar response = xmlHttp.responseText;// server error?if (response.indexOf("ERRNO") >= 0|| response.indexOf("error:") >= 0|| response.length == 0)throw(response.length == 0 ? "Server error." : response);// get response in XML format (assume the response is valid XML)responseXml = xmlHttp.responseXML;// get the document elementxmlDoc = responseXml.documentElement;result = xmlDoc.getElementsByTagName("result")[0].firstChild.data;fieldID = xmlDoc.getElementsByTagName("fieldid")[0].firstChild.data;// find the HTML element that displays the errormessage = document.getElementById(fieldID + "Failed");// show the error or hide the errormessage.className = (result == "0") ? "error" : "hidden";// call validate() again, in case there are values left in the cachesetTimeout("validate();", 500);}// sets focus on the first field of the formfunction setFocus(){document.getElementById("txtUsername").focus();}9. It's time to add the business logic now. Start by creating config.php, with this codein it:10. Now create the error handler code in a file named error_handler.php: