Para mim - To me (Formal way)
Pra mim - Informal way, this is the same as You too --- U2... People "cut" this, people create new things with the words, here in Brazil we do the same.
Por mim -- When we ask something we can use this: Você pode fazer isto POR MIM? (Could you do this FOR ME?)
I also can use this in sentences where something was made for me, as Lucas told you.
We just "cut" some words here in our daily life.
Like in USA people do the same: For you, you wrote: For U.
Or: ASAP ... (As soon as possible).
We also do the same here with these words:
Você ---- vc --- (You)
Também --- Tbm --- (Also)
Por que, porque, por quê e porquê --- Pq --- (Why/ Because)
Beijo --- Bj --- (Kiss) - We use this online to say "bye".
Daily we DON'T use for example: Eu ESTAVA (I was...) We USE: Eu tava.
but this is just our way to speak. To write "TAVA" it's wrong.
When you are writing something, you must write: ESTAVA.