//在线人数
function online_users() {
    $filename='online.txt';
    $CookieName='OnLinePerson';
    $onlineTime=30;
    $online=file($filename);
    $nowtime=$_SERVER['REQUEST_TIME'];
    $nowonline=array();
    foreach($online as $line){
        $row=explode('|',$line);
        $sesstime=trim($row[1]);
        if(($nowtime - $sesstime)<=$onlineTime){
            $nowonline[$row[0]]=$sesstime;
        }
    }
    if(isset($_COOKIE[$CookieName])){
        $uid=$_COOKIE[$CookieName];
    }else{
        $vid=0;
        do{
            $vid++;
            $uid='U'.$vid;
        }while(array_key_exists($uid,$nowonline));
        setcookie($CookieName,$uid);
    }
    $nowonline[$uid]=$nowtime;
    $total_online=count($nowonline);
    if($fp=@fopen($filename,'w')){
        if(flock($fp,LOCK_EX)){
            rewind($fp);
            foreach($nowonline as $fuid=>$ftime){
                $fline=$fuid.'|'.$ftime."\n";
                @fputs($fp,$fline);
            }
            flock($fp,LOCK_UN);
            fclose($fp);
        }
    }
    echo "$total_online";
}

<?php echo online_users() ?>

<p><?php echo online_users() ?> 位小伙伴正在浏览</p>