php如何实现和精确统计在线人数功能

echo session_id();
//6ed364143f076d136f404ed93c034201<br />

bool session_set_save_handler ( callable $open , callable $close , callable $read , callable $write , callable$destroy , callable $gc )
//callable 可随时支取的，请求即付的，随时可偿还的
// open(string $savePath, string $sessionName) 打开连接
//close() 关闭连接
//read(string $sessionId) 对出数据
//write(string $sessionId, string $data) //写入数据
//destroy($sessionId) //删除数据
//gc($lifetime) //垃圾回收函数

session_set_save_handler(
   array("session","open"),
   array("session","close"),
   array("session","read"),
   array("session","write"),
   array("session","destroy"),
   array("session","gc")
);

//链接数据的open
function open($path,$sessname) {
  $db = mysql_connect("localhost","root","123456","test");
  mysql_select_db("test",$db);
  mysql_query("SET NAMES UTF8");
  return true;
}

function close(){
$db = mysql_connect("localhost","root","123456","test");
mysql_close($db);
return true;
}

function read($sid){
  $sql = "select data from session where sid='{$sid}' and card='".self::$card."'";
  $query = mysql_query($sql) or die(mysql_error());
  $row = mysql_fetch_array($query);
  $row>0?$row["data"]:" ";
}

function write($sid,$data)
{
   $sql = "select sid from session where sid='{$sid}' and card='".self::$card."'";
   $query = mysql_query($sql) or die(mysql_error());
   $mtime = time();
   $num = mysql_num_rows($query);
   if($num){
    $sql = "UPDATE session SET data='{$data}', mtime ='{$mtime}'";
   }else{
    $sql = "INSERT INTO session (sid,data,mtime,ip,card) VALUES('{$sid}','{$data}','".time()."','{$_SERVER['REMOTE_ADDR']}','".self::$card."')";
   }
   mysql_query($sql);
   return true;
}

function destroy($sid){
  $sql = "DELETE FROM session WHERE sid='{$sid}'";
  mysql_query($sql) or die(mysql_error());
  return true;
}
function gc($max_time){
  $max_time = 600;
  $sql = "DELETE FROM session WHERE `mtime`<&#39;".(time()-$max_time)."&#39;";
  mysql_query($sql) or die(mysql_error());
  return true;
}

<?php
$filename=&#39;online.txt&#39;;//数据文件
$cookiename=&#39;VGOTCN_OnLineCount&#39;;//cookie名称
$onlinetime=600;//在线有效时间，单位：秒 (即600等于10分钟)
  
$online=file($filename);
//PHP file() 函数把整个文件读入一个数组中。与 file_get_contents() 类似，不同的是 file() 将文件作为一个数组返回。数组中的每个单元都是文件中相应的一行，包括换行符在内。如果失败，则返回 false
$nowtime=$_SERVER[&#39;REQUEST_TIME&#39;];
$nowonline=array();
//得到仍然有效的数据
foreach($online as $line){
  $row=explode(&#39;|&#39;,$line);
  $sesstime=trim($row[1]);
  if(($nowtime - $sesstime)<=$onlinetime){//如果仍在有效时间内，则数据继续保存，否则被放弃不再统计
    $nowonline[$row[0]]=$sesstime;//获取在线列表到数组，会话ID为键名，最后通信时间为键值
  }
}
/*
@创建访问者通信状态
使用cookie通信
COOKIE 将在关闭浏览器时失效，但如果不关闭浏览器，此 COOKIE 将一直有效，直到程序设置的在线时间超时
*/
if(isset($_COOKIE[$cookiename])){//如果有COOKIE即并非初次访问则不添加人数并更新通信时间
  $uid=$_COOKIE[$cookiename];
}else{//如果没有COOKIE即是初次访问
  $vid=0;//初始化访问者ID
  do{//给用户一个新ID
    $vid++;
    $uid=&#39;U&#39;.$vid;
  }while(array_key_exists($uid,$nowonline));
  setcookie($cookiename,$uid);
}
$nowonline[$uid]=$nowtime;//更新现在的时间状态
//统计现在在线人数
$total_online=count($nowonline);
//写入数据
if($fp=@fopen($filename,&#39;w&#39;)){
  if(flock($fp,LOCK_EX)){
    rewind($fp);
    foreach($nowonline as $fuid=>$ftime){
      $fline=$fuid.&#39;|&#39;.$ftime."\n";
      @fputs($fp,$fline);
    }
    flock($fp,LOCK_UN);
    fclose($fp);
  }
}
echo &#39;document.write("&#39;.$total_online.&#39;");&#39;;