How do you create a multilogin system using php?

[Peter]

You need to create a single page with a main IF statement which allows the boss to:
• if the form has not been posted the display it with fields to:
• type in his user name and password
• type in any new user name and password ready to be added to the file
• if the form has already been posted then act on it:
• check the user name and password against the existing list of acceptable
users and passwords in the text file
• if the details match then check to see if a new user is to be added:
• if so then add the user and display a message confirming that
• if not then just send a greeting message