The php code:
<?php
  $path = "./counter.txt";
  $count = intval(file_get_contents($path));
  if(isset($_GET["add"])) {
    $count++;
    file_put_contents($path, $count);
  }
  echo $count;
?>
To add one to the counter must be called with GET parameter add.
To get the counter value must be called with no parameters.
done_