Работа с файлами php

Не видит файл.

Warning:file_put_contents(test.txt): failed to open stream: Permission denied in /var/www/html/index.php on line 15

<?php
 error_reporting(-1);


 $counter_file = 'test.txt';

 if (!is_file($counter_file)) {
   $counter = 0;
 } else {
   $counter = file_get_contents($counter_file);
 }

 $counter++;

file_put_contents($counter_file, $counter);

?>