The PHP function SESSION_REGISTER() has been deprecated as of PHP version 5.4.
Instead, use the functin $_SESSION() will do.
// Use of session_register() is deprecated
$hello = "How are you.";
session_register("hello");
// Use of $_SESSION is preferred, as of PHP 5.4 and above
$_SESSION["hello"] = "Hello world, how are you.";
You may refer to the link below fore more information.
http://php.net/manual/en/function.session-register.php