bugs since I upgraded to wordpress 4.6.1

I have 2 sites on the same theme. One is running wordpress 4.4.5 and I upgraded one to wordpress 4.6.1. After the upgrade I started getting warnings on my site. The other site is fine. It seems like upgrading wordpress is just to much of a hassle. I noticed so many others have the same problems with the Illegal string offset and probably don’t even know it.

Warning: Illegal string offset ‘id’ in \
$do_not_duplicate = $post[‘ID’];

Warning: Cannot modify header information – headers already sent by