gcc Auto-Vectorization - Understanding output messages

I'm currently trying to understand the output of the gcc Vectorizer.I compiled my program using -O2 -ftree-vectorize -fopt-info-vec-all and gcc 8.2.0.

However, I do not understand, what is meant by some of the output messages, and cannot seem to find explanations on the internet.

What is meant by PHI in the following examples?

test.c:14: note: Analyze phi: i_53 = PHI <i_18(7), 0(5)>

test.c:14: note: Access function of PHI: {1024, +, 4294967295}_2

And what is the problem here?

test.c:5: note: not vectorized: not enough data-refs in basic block.

Any help is greatly appreciated.

(I'm not looking for help in solving the issues atm, just trying to understand what they are in the first place)