Topic Tag: Integrating with your own theme

Problem: I have Layout A assigned as the Layout for the Pages post type. However, under certain circumstances I would like to preselect Layout B when a Page is being created in wp-admin.

Solution: There's not a filter available to manipulate the selected Layout here, but you could use the WordPress "save_post" hook to override the User's Layout selection when the post is saved.

function override_default_layout( $post_id ) {
    if( get_post_type($post_id) == 'page' && wp_get_post_parent_id( $post_id ) == 1234 ) {
           // this page is a child of the page with ID 1234 so we need to update the _layouts_template postmeta
           update_post_meta( $post_id, '_layouts_template', 'template-slug');
        }
}
add_action( 'save_post', 'override_default_layout', 100 );

Change '1234' to match the numeric ID of the Photos page, and change 'template-slug' to match the slug of the correct Layout. This way, no matter what is selected, the correct Layout will be applied.

Relevant Documentation: https://codex.wordpress.org/Plugin_API/Action_Reference/save_post