How do I do this? Involves files.

Hi!! How would I do this!! I have a chat script and when someone logs in it copys their username to a txt file. It does each username on a different line. The problem is, I want to print each username on a different line. It may seem like a dumb question but I am new to PHP. How would I do this? Another problem is. When the user logs out of the chat I would like to delete that users username from the file. How would I do this. Please someone help. I am going crazy. Any help appreciated. Thanx Martin