Implementing an Ajax Call

<span style="float:left;">Username:</span>
<span id="showajax" style="display:none;float:right;">
<img src="/images/ajax-pump.gif" alt="" style="float:left" /></span>
<input
type="text"
name="username"
maxlength="50"
size="40"
onclick="clearnameresults();"
onblur="clearnameresults(); checkuser(this.value);"
/>
<span id="ajaxresult"></span>
<script type="text/javascript" src="include/ajax.js"></script>
<script type="text/javascript" src="include/checkuser.js"></script>

<script type="text/javascript">
function clearnameresults() {
    clearspan('ajaxresult');
}
</script>

var in_param = '';
var in_param2 = '';
function checkuser(username) {
    if(document.frmregister.username.value != '') {
        document.getElementById('showajax').style.display = 'block';
        var data = MakeNewAJAXCall("checkuser.php?u="+username, SimpleCallback, 'GET', ComplexCallback, in_param, in_param2, in_timeout_callback, 7);
    }
}
function SimpleCallback(in_text, in_param, in_param2){
    if(in_text != "") {
        if(in_text == "OK") {   
            //Ok this username is safe so
            document.getElementById('ajaxresult').innerHTML = 'Username available';
            document.getElementById('ajaxresult').style.color = 'green';
        } else {
            //... do something else...
        }
        document.getElementById('showajax').style.display = 'none';
    }
}

function clearspan(ajaxspan) {
    document.getElementById(ajaxspan).innerHTML = '';
}

function choosealternative(names) {
    var uname = document.frmregister.username.value;
    var ret = '<table>';
  var i=0; var x = 0;
  var n = '';
  var randomnumber = 0;
  for(i=0;i<3;i++) {
    randomnumber=Math.floor(Math.random()*9999)
    n = uname + randomnumber;   
      ret += '<tr><td><input onclick="document.frmregister.username.value = this.value;document.frmregister.username.focus();" type="radio" name="altname" value="' + n + '" /></td><td>' + n + '</td></tr>';
    }
    ret += '</table>';
    return ret;
}

function ComplexCallback(in_stage, in_text, in_param, in_param2, in_index) { return true;}
function in_timeout_callback() {alert('timed out, please check your Internet connection');}

<?php
/*******************************************************************
 * Ajax calls to check username is available
 *******************************************************************/

require_once('include/common.php');

if(!isset($_GET['u'])) die('No params sent');

echo is_available(htmlspecialchars(strtolower($_GET['u']))) ? "OK" : "TAKEN";

function is_available($username) {
    $sql = "SELECT id FROM users WHERE LCASE(username)='$username'";
    $result = mysql_query($sql) or errorlog( __LINE__."\n\n".mysql_error()."\n\n$sql");
    if($result && mysql_num_rows($result)>0) {
        return false;
    } else {
        return true;
    }
}

?>