php实现简单聊天功能

CREATE TABLE `guanhui`.`message` ( 
    `id` INT(10) NOT NULL AUTO_INCREMENT COMMENT '消息ID' , 
    `content` VARCHAR(255) NOT NULL COMMENT '消息内容' , 
    `sender` VARCHAR(60) NOT NULL COMMENT '发送者' , 
    `send_time` INT(10) NOT NULL COMMENT '发送时间' ,
     PRIMARY KEY (`id`)
 ) ENGINE = MyISAM;

$con = mysql_connect("localhost","","");
if (!$con) {
    die('Could not connect: ' . mysql_error());
}
mysql_select_db("chat", $con);
mysql_query("set names 'utf8'");            
$sql = "SELECT * FROM  `message`";
$result = mysql_query($sql);
if(isset($result)){
    json_encode(array(
        'code' => 0,
        'msg' => 'OK',
        'data' => mysql_fetch_array($result, MYSQL_ASSOC)
    ))
} else {
    json_encode(array(
        'code' => 0,
        'msg' => '聊天信息为空！'
    ))
}

setInterval(function(){

    $.get('/get_message_list.php', function(data){

        $(data).each(function(value, item){
            $('#message_list').append('来自：' + data.sender + '的消息：' + data.message + '发送时间：', data.send_time);
        });

    });

}, 600);

$send_time = date('Y-m-d H:i:s',time());;
$send = $_POST['sender'];
$content = $_POST['content'];
$con = mysql_connect("localhost","","");
if (!$con) {
  die('Could not connect: ' . mysql_error());
}
mysql_select_db("ing", $con);
mysql_query("set names 'utf8'");
$insert="INSERT INTO  `chat` (`id` ,`sender` ,`content` ,`send_time`) VALUES (NULL ,  '$sender',  '$content',  '$send_time')";
$result = mysql_query($insert);

function sendcontent()
  {
    var xmlhttp;
    if (window.XMLHttpRequest)
    {// code for IE7+, Firefox, Chrome, Opera, Safari
      xmlhttp=new XMLHttpRequest();
    }
    else
    {// code for IE6, IE5
      xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.onreadystatechange=function()
    {
      if (xmlhttp.readyState==4 && xmlhttp.status==200)
      // 服务端返回了正确数据，开始响应处理
      {   
        document.getElementById("input").innerHTML="";
      } 
    }
    xmlhttp.open("POST","/send_message.php",true);
    var f=document.chat;
    var content = f.content.value;
    var sender = f.sender.value;
    //发送请求
    //这里使用Post方法传递参数；
    //将要构造的参数连接起来，接收的时候：$_POST['send'];
    var post_str= "content="+ content+"&sender="+sender;
    //使用post的时候必须在发送请求之前加上下面这句
    xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
    xmlhttp.send(post_str);
  }