Okay, the good news first:
Your notice don't have any effect on the functionality of your code.
Thats just a message from your PHP Parser that you want to use a variable (or in your case a index of an array) which isn't set at the moment.
In your code thats the point, if you haven't sended your register form.
You could redefine the error_reporting of your PHP to hide this messages, or you could tweak your code a little bit to get rid of them (which i wouldn't recommend if you want to follow a tutorial).
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