if you are creating a php page with the following code:

<?php // Show all information, defaults to INFO_ALL phpinfo(); ?>

and you view the page, and do not have a section named "session" as shown below:

Go to /usr/lib/php/ ... is there a session.so file in there?

Unless there is some funky file/folder permission issue here... I really don't know. If you installed session module it should be enabled by default.

I guess you could try getting rid of everything php and doing an opkg update and then re-install php7 php7-cgi php7-mod-session... I don't know what else to suggest.