<?php
/* Created on [2012-5-21] Author[Newton] */
?>

  <textarea name="message" rows="10" cols="50" onclick="checkUser()" wrap="off">
  Give message please.
  </textarea>

  <script language="JavaScript" type="text/javascript">
//Ajax
var xmlHttp;

    function createXMLHttpRequest() {
        if(window.XMLHttpRequest) {
            xmlHttp = new XMLHttpRequest();
        } else if (window.ActiveXObject) {
            xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
    }

    function checkUser(){
        createXMLHttpRequest();
        url = "action.php?check=ok&ran="+Math.random();
        method = "GET";
        xmlHttp.open(method,url,true);
        xmlHttp.onreadystatechange = show;
        xmlHttp.send(null);
    }

    function show(){
        if (xmlHttp.readyState == 4){
            if (xmlHttp.status == 200){
                var text = xmlHttp.responseText;
                alert(text);exit;
                //document.getElementById("s2").innerHTML = text;    //可将返回信息放入id为s2的div中
            }else {
                alert("response error code:"+xmlHttp.status);
            }
        }
    }
  </script>

#action.php
if(isset($_GET['check'])){
    #mysql执行语句,检测用户是否存在
    echo "检验中……";
}

------解决方案--------------------

探讨


ajax来实现。onfocus触发ajax求请求php检查是否登录。