Out of two fresh installs I've observed the Piwik port didn't work without the PHP cli option enabled.
The Piwik port did not check wether the cli version of PHP is installed.
If you use the crontab method to generate the output (in large environments prefered over at-visit-generated), then you certainly would need it.

Further php5-session is also required, when it's missing you'll end up with a fatal error, even before any authentication is setup.

And 3th: php5-json is needed to come to the full result.

Fix: 

It's easy to fix manually if you're aware of the issue.
However, I think it's strange to have halfworking ports while they can be better.

I don't know how ports are made.
But it should somehow do a check to see if PHP works in the shell.


For the other two problems, -I asume- in the Makefile, the line:

USE_PHP=	mysql gd pdo pdo_mysql zlib iconv dom

should be

USE_PHP=	mysql gd pdo pdo_mysql zlib iconv dom session json
How-To-Repeat: on a system without any PHP yet,
install PHP with CLI disabled,
then install the Piwik port.