A session-based visitor counter

gistfile1.php

<?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 "You are visitor number $counterVal to this site";

I can't get the output sir..the counter was not incremented

Oh dear. Is a file being created? If not then it's probably a permissions issue.
Try changing the owner of the folder to www-data, e.g. sudo chown -R www-data:www-data /var/www/html/counter
Other than that, remember that the counter is only meant to increment once per session, not per page load.
If that's the case, try opening the page in two different browsers.