broken I phone came alive (well, partly at least)

two years ago somebody gave me there iPhone 4 that was dropped into a toilet and never gave any sign of live ever since. she wanted to true it away and i just said to give it to me since i just like to fool around with old or broken electronics and search for what still can be used.

i forget about it, but came across it two days ago, and thought "maybe all the water dried out of this thing, maybe it will work",i didn't had any hopes what so ever for it, but i had a charging cable laying around and plugged it in "nothing to loose".

after a few minutes i heard and felt it vibrate. i unplugged it, plugged it back in and again, after a few minutes it vibrated. i left it charging for a few hours when i came back i unplugged it ,tried to turn it on, but no reaction. when i plugged it in again i made this "ploing" sound. i decided to let it charge a while longer.

now it vibrates when i turn off the silent mode, it makes the sound when i plug the charger in, but the screen doesn't do anything really. unless i try to force restart the phone then i see, very weak the apple logo.

what does this mean? is only the screen dead? would the phone be "normal" again when it is replaced? or is it something else?

can somebody please help to diagnose this iphone further?