When you change the php.ini you need to restart Apache.
What you are probably experienceing is the CookieMonster problem, not the session problem.
TikiWiki 1.8 uses a lot of cookies, and the browser will only allow 20 maximum. Each submenu item on the menu uses a cookie, not to mention about 4 others for the site setup, plus any others for extra menus and tabs, etc that you use.
So you will probably need to remove the menu options, or redesign without having submenus.
There are better fixes in 1.9, but that code is not stable yet.