A more accurate PHP code that counts the number of people online

$filename='online.txt';//Data file
$cookiename='VGOTCN_OnLineCount';//Cookie name
$onlinetime=600;//Online valid Time, unit: seconds (that is, 600 equals 10 minutes)
$online=file($filename);
//PHP file() function reads the entire file into an array. Similar to file_get_contents(), except file() returns the file as an array. Each cell in the array is a corresponding line in the file, including newlines. If it fails, return false
$nowtime=$_SERVER['REQUEST_TIME'];
$nowonline=array();
//Get the data that is still valid
foreach($online as $ line){
 $row=explode('|',$line);
 $sesstime=trim($row[1]);
 if(($nowtime - $sesstime)<=$onlinetime){//if If it is still within the valid time, the data will continue to be saved, otherwise it will be abandoned and no longer counted
 $nowonline[$row[0]]=$sesstime;//Get the online list into the array, the session ID is the key name, and the last communication time is the key Value
 }
}
/*
@Create visitor communication status
Use cookies to communicate
COOKIE will become invalid when the browser is closed, but if the browser is not closed, this COOKIE will remain valid , until the online time set by the program times out
*/
if(isset($_COOKIE[$cookiename])){//If there is a COOKIE, that is, it is not the first visit, the number of people will not be added and the communication time will be updated
 $uid=$_COOKIE[$ cookiename];
}else{//If there is no COOKIE, it is the first visit
 $vid=0;//Initialize the visitor ID
 do{//Give the user a new ID
 $vid++;
 $uid='U'. $vid;
 }while(array_key_exists($uid,$nowonline));
 setcookie($cookiename,$uid);
}
$nowonline[$uid]=$nowtime;//Update the current time status
//Count the number of people online now
$total_online=count($nowonline);
//Write data
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($fp);
 }
}
echo 'document.write("' .$total_online.'");';
?>