HTML
<script>
function createRequestObject() {
    var ro;
    var browser = navigator.appName;
    if(browser == "Microsoft Internet Explorer"){
        ro = new ActiveXObject("Microsoft.XMLHTTP");
    }else{
        ro = new XMLHttpRequest();
    }
    return ro;
}

var http = createRequestObject();

function sndReq(zip) {
    http.open('get', 'zipcode.php?zip='+zip);
    http.onreadystatechange = handleResponse;
    http.send(null);
}

function handleResponse() {
    if(http.readyState == 4){
        var response = http.responseText;
        var update = new Array();

        if(response.indexOf('|' != -1)) {
            update = response.split('|');
            document.getElementById("city").value = update[0];
   document.getElementById("state").value = update[1];
        }
    }
}
</script>
<h3>Enter A United States Zipcode, Then Tab</h3>
<table align="center">
  <tr>
    <td>Enter Zipcode:</td>
    <td><input type="text" id="zipcode" name="zipcode" onBlur="sndReq(this.value);"/></td>
  </tr>
  <tr>
    <td>City:</td>
    <td><input type="text" id="city" name="city"/></td>
  </tr>
  <tr>
    <td>State:</td>
    <td><input type="text" id="state" name="state"/></td>
  </tr>
</table>

<?php
$dbuser = 'root';
$dbpass = '111111';

$cn = mysql_connect("localhost", $dbuser, $dbpass);
$db = mysql_select_db("ajax");

$sql = "select city, state from zipcodes where zipcode = " . $_REQUEST['zip'];
$rs = mysql_query($sql);
$row = mysql_fetch_array($rs);

echo $row['city'] . "|" . $row['state'];

mysql_close($cn);
?>

if(response.indexOf('|' != -1)) {
            update = response.split('|');
            document.getElementById("city").value = update[0];
            document.getElementById("state").value = update[1];
}