Please note that the 3rd argument is really a "path".
<?php
imagettfbbox(10, 0, 'arial.ttf', 'Hello, World!'); // will result in "Warning: imagettfbbox(): Could not find/open font in ...php on line ..."
?>
use instead something like this:
<?php imagettfbbox(10, 0, './arial.ttf', 'Hello, World!'); ?>
or
<?php imagettfbbox(10, 0, getcwd().'/arial.ttf', 'Hello, World!'); ?>