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//This code checks for a featured image when updating. If no image exists it will save the current draft without publishing, and display an error message to the writer. This will ensure that no post can go live without a featured image attached.

add_action('save_post', 'wpds_check_thumbnail');
add_action('admin_notices', 'wpds_thumbnail_error');
function wpds_check_thumbnail($post_id) {
    // change to any custom post type
    if(get_post_type($post_id) != 'post')
        return;
    if ( !has_post_thumbnail( $post_id ) ) {
        // set a transient to show the users an admin message
        set_transient( "has_post_thumbnail", "no" );
        // unhook this function so it doesn't loop infinitely
        remove_action('save_post', 'wpds_check_thumbnail');
        // update the post set it to draft
        wp_update_post(array('ID' => $post_id, 'post_status' => 'draft'));
        add_action('save_post', 'wpds_check_thumbnail');
    } else {
        delete_transient( "has_post_thumbnail" );
    }
}
function wpds_thumbnail_error()
{
    // check if the transient is set, and display the error message
    if ( get_transient( "has_post_thumbnail" ) == "no" ) {
        echo "<div id='message' class='error'><p><strong>You must select Featured Image. Your Post is saved but it can not be published.</strong></p></div>";
        delete_transient( "has_post_thumbnail" );
    }
}