since you have a phpBB3 that works, than your php extension is loaded and being used
A setting I use myself in the php.ini (look for disable_functions) is:
disable_functions = phpinfo
Phpinfo is then disabled via the server and returns a blank page when it is called in a page as <?php phpinfo(); ?> or whatever variation of the phinfo command.
I use this setting to improve security, visitors don't need to know which version of php I'm running and / or php extensions enabled.
Check if this setting is on on your server, this would explain why it doesn't display the information.
If you're on a hosted service, than your provider might have set this setting => one has to enabled it, by default disable_functions is empty and commented out in the php.ini.