phpbb_root simply means the directory where your board is installed.
if your board is installed in a folder named phpBB3 then that is what root means.
if your board is installed in a folder named forum, then forum is your root.
if your board is installed in you public_html directory then that is your root.
and , the only extensions that go into the ext/phpbb folder are the official phpbb.com extensions. there are only a few of those so far.
all the rest have to go into a folder of their own under the ext folder as shown above.
I am going to be out of town and off line for a week . see ya when I get back.
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