Error messages

[KeithAus]

Recently I updated from Xampp 1.7.3 to 1.7.4, when I go into a login page on my site (written in PHP) i get an error message “Notice: Undefined index: submitbutt in C:\xampp\htdocs\bazaar\login.php on line 22” this message is in the body of the code

If I enter a user name that is in my database, it allows me to log in, but i then get 2 error message appearing above the header:
1"Notice: Use of undefined constant myusername - assumed ‘myusername’ in C:\xampp\htdocs\bazaar\purchase.php on line 4"
And
2"Deprecated: Function session_is_registered() is deprecated in C:\xampp\htdocs\bazaar\purchase.php on line 4"

Until I updated Xampp, I had none of these code errors. Can someone help out please?

[KeithAus]

The code for the login page is as follows:

Login

<?php include ("header.php"); ?>

.: Login :.

Existing Member
<?php if($_POST['submitbutt']) { echo "myusername: ". $_POST['UserName'] ."
"; echo "mypassword: ". $_POST['Password'] ."
"; } else { ?>

User Name:

Password:

<?php } ?>

<?php include ("footer.php"); ?>

And the code for the purchase page is as follows:

<?php // Check if session is not registered , redirect back to main page. // Put this code in first line of web page. session_start(); if(!session_is_registered(myusername)) ?> Purchase

<?php include ("headerli.php"); ?>

.: Purchase :.   Welcome, <?php echo $_SESSION["UserName"]?>, you are now logged in!

To make a purchase simly click the "add to cart" button and then select the "purchase" button when you have completed your shopping

<?php include ("footer.php"); ?>