wont really effect anything, that if statement is just comparing the two strings "checkout_failured.php" and "".
to make sure that a failure page has actually been set. If one where not sset, thaen the else if would be:
else if ("" != "") {
header("Location: ". "");
exit();
}
it is really there to prevent:
header("Location: ". "");
from being run is a failure page had not been chosen, which would generate a PHP error.