<script type="text/javascript" src="jiance.js"></script>
<form name="myform" action="" method="get">
 用戶名：<input name="user" value="" type="text" onblur="funtest100()" />
 <div id="test100"></div>
</form>

var xmlHttp;
function S_xmlhttprequest(){
  if(window.ActiveXobject){
    xmlHttp = new ActiveXObject('Microsoft.XMLHTTP');
  }else if(window.XMLHttpRequest){
    xmlHttp = new XMLHttpRequest();
  }
}
function funtest100(){
  var f = document.getElementsByTagName_r('form')[0].user.value;//獲取文本框內(nèi)容
  S_xmlhttprequest();
  xmlHttp.open("GET","jcfor.php?id="+f,true);//找開請(qǐng)求
  xmlHttp.onreadystatechange = byphp;//準(zhǔn)備就緒執(zhí)行
  xmlHttp.send(null);//發(fā)送
}
function byphp(){
  //判斷狀態(tài)
  if(xmlHttp.readyState==1){//Ajax狀態(tài)
    document.getElementByIdx_x_x('test100').innerHTML = "正在加載";
  }
  if(xmlHttp.readyState==4){//Ajax狀態(tài)
    if(xmlHttp.status==200){//服務(wù)器端狀態(tài)
      var bytest100 = xmlHttp.responseText;
      //alert(bytest100);
      document.getElementByIdx_x_x('test100').innerHTML = bytest100; 
    }  
  }
}

<?php
 if($_GET[id]){
    sleep(1);
    $conn=mysql_connect('localhost','root','');
    mysql_select_db('test',$conn);
    $sql="SELECT * FROM `user` WHERE `name`='$_GET[id]'";
    $q=mysql_query($sql);
 
    if(is_array(mysql_fetch_row($q))){
      echo "用戶名已經(jīng)存在"; 
    }else{
      echo "用戶名可以使用"; 
    }
 }  
?>