Help me - Flash save to .txt using php

[grimm 88]

hi, i really need help with understanding this, and before i get a load of replys saying 'use the search function' or whatever, let me tell you that i already did... i found a few examples but i am a total noob to php

so what i need is... somebody who can either tell me step by step instructions on how to write the php and actionscript required for this to work OR an example with comments (preferably someone who can explain it to me though )

anyway... here is a little more background on what i am trying to achieve:

i want to have a user creation page where they enter a username and password, the php then loads up the .txt file and writes the new user into it with something like:

username=harry983
harry983_password=harrypassword

then at the login screen, it first checks wether the username that the user has entered is valid and matches with one in the text file, and if it is not found in the text file stop and say 'username invalid' in a text box, if it is found, check to see wether the password they entered matches username_password, if it doesn't match display 'password invalid' in a text box, and if it is correct then it should load other pieces of data like username_age

anyway... thats basically what i am trying to achieve and i know loads of flash... its just the php and the communication actionscrpit between flash and the php file that is confusing me