I want to manipulate a $_POST via PHP

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Is there anyone that can help me with this I don't know where to begin. I want to encrypt a password $_POST and save it to a file. Is there anyway I can make $_POST equivalent to a php array.

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Ok I learned that you can access parts of a $_POST[][] but I still have some problems I need help with. I want to make a function that will handle the array $_POST["input"][intiger] but I dont know how to declare a function that will accept $_POST. If anyone here wants to demonstrate something for my password hashing that deals with arrays that would be help also. Like how do you know when the array $_POST[][] is at its last variable or do I have to use sizeof $_POST to determine the size of the post in my code. Can all variables be accessed like they where arrays?

I know you can declare a function like below.

function password_hash($password){}

But I want to declare a function with an array and I am not sure how to.
I was hoping someone could clarify things for me on this subject.

function password_hash($password[]){}

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Ok if you read both of the posts I wrote and have gotten this far then don't be upset but I have tested and found the answer to all of my questions unto a certain degree celsius. I figure if you wanted to help me you still can. If you don't you don't have to. I would love to see some php code that I could use to hash a password. Also I still haven't found the terminating array character.