How do I add an if / else?

I am using the following code to pass a variable. if variable = a, do nothing.

I then want to check if variable = a, do nothing, if b, do nothing, else do something

<?
    if($_GET['pageid'] == 'a'){
    } else {
    include('header_image.php');
    }   
?>

Above is the code I have working correctly for one vartiable.

How do I add an if / else?