Strange 'index.php' Issue
This recently tripped me up. 'true' (with quotes) is not the same as true (without the quotes).

Lose the quotes in the config file, clear Craft's caches, and it should stick.

Also make sure your siteUrl is the same as what your web server is configured as. If you have a redirect in there, say www.example.com goes to example.com, and your siteUrl is configured to use the former, that seems to trip up Craft's check too.