Set post featured image to author image

Question

How would I set the featured of image of a post to be the image (an ACF User field) of the post author so that anywhere my theme uses the featured image, the author's image appears instead.

I'm aware of set_post_thumbnail, but I'm not sure which action or filter to use to make the change globally.

Answer 1

If you use the save_post hook, every time content is published or updated, the featured image will be refreshed. Note: make sure ACF is set to return an image ID, which is not its default.

add_action( 'save_post', 'wpse_set_featured_image' );
function wpse_set_featured_image($post_id) {
    // get author id
    $author_id = get_the_author_id($post_id);
    // get author's image
    $author_image_id = get_user_meta($author_id, 'your_acf_img_var_name', true);
    // fallback image
    if(empty($author_image_id)) {
        // set to an existing image ID to use as a fallback
        $author_image_id = '4';
    }
    // at last, set the post featured image
    set_post_thumbnail($post_id, $author_image_id);
}