Answers for "php cant rename files using variables"

rename file php

rename('old_filename.txt', 'new_filename.txt');

php rename files in directory

<?php rename ("/folder/file.ext", "newfile.ext"); ?>

The above doesn't rename the file within the folder, as you might assume, 
instead, it moves the file to whatever the PHP working directory is... 
Chances are you'll not find it in your FTP tree. 
Instead, use the following:

<?php rename ("/folder/file.ext", "/folder/newfile.ext"); ?>