$_GET
This is the GET Property of the PHP Script that returns Value from the Form by using the GET Value property.
['act'] == 'register'
This is the parameter that is passed back to the PHP Page. This implies register.php?act=register
$user, $pass1,$pass2
These are the variables in which the User ID, Password and Confirm Passwords are stored.
anti_injection()
This is a function that is defined to prune the special characters in the User ID or password inorder to prevent SQL Injection in the script.
$_POST[]
This is the Form POST command that will return the values from the Text boxes once the submit button is pressed.
login, senha1, senha2
These are the names of the Text Boxes which take the Input from the user.
There is NO Tutorial. All that you need to do is "LEARN PHP FIRST"