When you see the php file via web browser, it will display something like:
/homepages/35/d120461223/htdocs/[your path to myBB installation]
This is the absolute path to use.
Good luck!
I was able to solve this problem. One way to know your absolute path is to use php function, realpath(). You can create a php file, include echo realpath(‘./’) in the file, and upload to your mybb directory.
Then type the file in your browser.
This is the code that I used to know the absolute path for mybb installation.
<?php echo realpath('./'); ?>
I hope this info is helpful.