I'm quite sure Syrena never ate Phillip...
Here's my theory (it's just a theory, but you'll see, it makes sense)
1-When Syrena gets a "totally human" form after her water cage broke down, Black Beard forces her to walk but she can't, Phillip kneels beside her and says:
"Put your arms around me."
"I do not ask for help!"
He then replies:
"But you need it."
And then only, she puts her arm around Phillip' shoulders...
That's the first part of my theory. Phillip helps Syrena even if she never ask for any help.
In my idea, the end of the movie is just the "mirror-scene" of the other one... Now Phillip needs help. Syrena arrives and says she can heal him "if he asks to"... but he only asks her to forgive him (so, he also doesn't ask for help, even if he REALLY needs it). She then kisses him (and it's been said ealier in the movie that a kiss from a mermaid protects from drowning) and drags him to a mysterious somewhere... She has to save him!
...and I don't say that because I want it, I'm quite a big cinema fan, and it always works that way: when something happens in one way, it will happen in the other way.
For example, in the old Star Wars trilogy, when Han Solo is about to be frozen in carbon, Leia says "I love you", and he answer "I know", in the next movie, when Leia and Han are threatned by Storm troopers at the bunker's door, Leia shows Han the gun she hided all that time... WHAT ARE THEY SAYING? Han:"I love you", and Leia:"I know"...
I have read some books about the teen Jack :DD And the 2nd book was about sirens-mermaids.It tells that that there is under water cave where all the mermaids-sirens is protecting their treasures and also lives there.So I think that Syrena got Philip there.
well i think she did save him because in the scene were the pirates were baits for the mermaids, one of the pirates said that if you were kissed by a mermaid you will never drowned. Serena kissed Phillip because he couldnt live on ground anymore because of his wound, so serena decided to kiss Phillip because she loves him and that its the only way to save him, so they lived happily ever after underwater lol