Problem

<?php
session_start();
$counter_name = "counter.txt";
// Check if a text file exists. If not create one and initialize it to zero.
if (!file_exists($counter_name)) {
  $f = fopen($counter_name, "w");
  fwrite($f,"0");
  fclose($f);
}
// Read the current value of our counter file
$f = fopen($counter_name,"r");
$counterVal = fread($f, filesize($counter_name));
fclose($f);
// Has visitor been counted in this session?
// If not, increase counter value by one
if(!isset($_SESSION['hasVisited'])){
  $_SESSION['hasVisited']="yes";
  $counterVal++;
  $f = fopen($counter_name, "w");
  fwrite($f, $counterVal);
  fclose($f);
}

echo "Liczba odwiedzin: $counterVal"; ?>