Set a featured image as a replacement to the header image

Question

I am setting a header image as a background image to the .blog__header block via inline styles in functions.php:

$header_image = get_header_image();

if ( $header_image ) {
    $header_image_css = "
            .blog__header {
                    background-image: url({$header_image});
            }";
    wp_add_inline_style( 'theme-styles', $header_image_css );
}

now I want to replace this header image with a featured image if set:

$header_image = get_header_image();

if ( has_post_thumbnail() ) {
    $post_thumbnail = get_the_post_thumbnail('full');
}

if ( $header_image ) {
    $header_image_css = "
            .blog__header {
                    background-image: url({$header_image});
            }";
    wp_add_inline_style( 'theme-styles', $header_image_css );
} elseif ( has_post_thumbnail() ) {
    $post_thumbnail_css = "
            .blog__header {
                    background-image: url({$post_thumbnail});
            }";
    wp_add_inline_style( 'theme-styles', $post_thumbnail_css );
}

however it doesn't work, probaly because I have not set the post ID. Any assistance would be greatly appreciated!

Answer 1

You did not mention or shown in the code to which hook are you hooking the above code to, but here's what I think should work. NOT TESTED

global $post;
$header_image = '';

// featured image is first priority, right?
if ( has_post_thumbnail( $post->ID ) ) {
    $header_image = get_the_post_thumbnail_url( $post->ID, 'full' );
} elseif ( !empty( get_header_image() ) ) {
    $header_image = get_header_image();
}

$header_image_css = ".blog__header { background-image: url({$header_image}); }";
wp_add_inline_style( 'theme-styles', $header_image_css );