Simple edit, that will automatically log the user in upon sign up, which will make user experience better, as they don't have to enter their details twice to play for the first time.
Open up register.php, and at the top, below line 1, add;
Now, find the following snippet; (Code Mucked Up?)
and directly below it add; (Code Mucked Up?)
If you get the following error; Cannot modify header information
And that's it.