Ajax PHP no refresh secondary linkage drop-down menu (province and city linkage) source code_PHP tutorial

ajax.js

var http_request = false;
function send_request(url,method) {//Initialization, designated processing function, function to send request
http_request = false;
//Start initializing the XMLHttpRequest object
if(window.XMLHttpRequest) { //Mozilla browser
http_request = new XMLHttpRequest();
if (http_request.overrideMimeType) {//Set MiME category
http_request.overrideMimeType('text/xml');
}
}
else if (window.ActiveXObject) { // IE
try {
http_request = new ActiveXObject("Msxml2.XMLHTTP");
} catch (e) {
try {
http_request = new ActiveXObject("Microsoft.XMLHTTP");
} catch (e) {}
}
}
if (!http_request) { //Exception, failed to create object instance
window.alert("Cannot create XMLHttpRequest object instance.");
return false;
}
switch(method){
case 1: http_request.onreadystatechange = processRequest1;break;//Select operation function
case 2: http_request.onreadystatechange = processRequest2;break;
case 3: http_request.onreadystatechange = processRequest3;break;
}
// Determine the method and URL of sending the request and whether to execute the next code synchronously
http_request.open("GET", url, true);
http_request.send(null);
}
// Function to process returned information
function processRequest1() {//Operating function 1, transfer into province
if (http_request.readyState == 4) { // Determine object status
if (http_request.status == 200) { // The information has been returned successfully, start processing the information
document.getElementById("statusTxt").innerHTML="";
addOptionGroup("province",http_request.responseText);
} else { //The page is abnormal
alert("There is an exception in the page you requested.");
}
}else {//As long as the reading is not completed
document.getElementById("statusTxt").innerHTML="Regular reading data...";
}
}

function processRequest2() {//Operating function 2, transfer into the market
if (http_request.readyState == 4) { // Determine object status
if (http_request.status == 200) { // The information has been returned successfully, start processing the information
document.getElementById("statusTxt").innerHTML="";
addOptionGroup("city",http_request.responseText);
} else { //The page is abnormal
alert("There is an exception in the page you requested.");
}
}else {//As long as the reading is not completed
document.getElementById("statusTxt").innerHTML="Regular reading data...";
}
}

function processRequest3() {//Operation function 3, enter province and city
if (http_request.readyState == 4) { // Determine object status
if (http_request.status == 200) { // The information has been returned successfully, start processing the information
document.getElementById("statusTxt").innerHTML="";
document.getElementById("district").value=http_request.responseText;
} else { //The page is abnormal
alert("There is an exception in the page you requested.");
}
}else {//As long as the reading is not completed
document.getElementById("statusTxt").innerHTML="Regular reading data...";
}
}

function loadProvince() {//Load province