Choose different header images on different posts

Together with the WordPress Customizer a nice feature was introduced into WordPress: The header image. If a theme supports header images with add_theme_support() you can choose an image, which will be displayed in the header. I’ve read now an interesting question in a forum:

Can someone please tell me if it is possible to alter the header image in the theme Twenty Eleven (2.1) for every page?

I liked this question and thought to myself, there must be a nice filter function, you could use for this. So I started my search. The image gets output by get_header_image(), which can be found in wp-includes/theme.php. Of course, this function is using get_theme_mod(), which is located in the same file. get_theme_mod() expects the name of the mod as a parameter, in our case “header_image”.

Before get_theme_mod() outputs its result, the function filters it with the filter “theme_mod_{name}”. For the image, which is displayed in the header, the filter would be called “theme_mod_header_image”. Basically we are done here. We write a small function which filters our header image. In a first step the function checks with is_singular() if we are on a single page or post. If that’s the case, we check if the current page has the custom field “current-background”. If this field contains a valid URL, this URL will be returned. In every other case, our filter will return the standard header image, which the administrator defined in the Customizer: