Message: mkdir(): Permission denied

You can use the PHP function is_writable( $filename )
http://php.net/manual/en/function.is-writable.php
It should check if the current user may write to the passed filename, where filename may be a file or directory.

But if I was you, I would create a directory manually in your root directory and set the read/write permissions. Then use this directory as your base dir for the uploads. As this directory seems to be required for your application there is no need to set it via a PHP script