Moving from PHP to C is definitely a tough gig, so you shouldn’t feel too discouraged. If you’re willing to tough it out, it’ll get better.
It’s difficult to predict what will work best for you, but I do think you should consider starting over and making sure you’re closer to 100% on each chapter before you move on. The error rate will pile up over time, and when you hit the more advanced stuff, you may be able to bluff your way through, but real understanding will be difficult.
Another reader asked for additional challenges, and I came up with a few; he then vanished to another board (no, I’m not bitter) but if you’d like, taking that approach might make sense. Work through an extra challenge or two for each chapter to give you a method for testing your success rate at learning the material will help keep you from moving too quickly.
Another option as you suggested (no, don’t laugh, and no, this is in no way an insult) is to pick up a used copy of C for Dummies (I rather liked the book, and kept waiting for the 2nd part; I think it did come out years later, but don’t recall off-hand) and work through that before coming back to this. But I think if you made it this far in the book before crying uncle, you’ve got a good chance at making it work, so I’d suggest rewinding a few chapters instead.