Problems with chmod()

[EDUARD59]

I am trying to change the file permissions of the file "contador.txt" with the following program:

<?php
error_reporting(E_ALL|E_STRICT);
ini_set('display_errors',true);
chmod("contador.txt",0777);
?>

The file contador.txt is the same directory of the script.
But I can't change the file permissions ant the following message appears:

Warning: chmod()😮peration not permitted in /usr/www/htdocs/eduardo/cambia.php on line 5

What can I do? Please help.

[scrupul0us]

the user apache/IIS is running under doesnt have sufficient user/group rights to change the permissions of the file

as a note, making files/directories 777 is terribly insecure