add_filter to the_content after apply_filters

Question

I have a shortcode that outputs null ("").

What I'm trying to accomplish is to output a default message if the shortcode outputs null. I don't want to extend my shortcode because there may be multiple instances of the shortcode and I don't want to output the message multiple times.

So my function is as follows:

function av_standalone_doc_404($content) {
    if(has_term('standalone-document', 'formats')){
        $filtered_content = apply_filters('the_content', $content);
        if(empty($content)){
            // Display 404 message
            $output .= '<div id="404_message">Error:
                Please check this page later.</div>';
        }else{
            $output .= '';
        }
        return $output . $content;
    }
}
add_filter('the_content', 'av_standalone_doc_404', 12);

The problem is that apply_filters is causing a fatal error - obviously because you can't apply a filter to the content and call apply_filters within that filter.

The '12' load order doesn't appear to help either.

How can I apply the filter after the subsequent filters which apply the shortcode to the_content?

Answer 1

if you want to keep the current setup, try to hook in early and use do_shortcode():

function av_standalone_doc_404($content) 
{
    remove_filter( current_filter(), __FUNCTION__ );

    if ( has_term( 'standalone-document', 'formats' ) )
    {
        $filtered_content = do_shortcode( $content );
        // …

If you want to avoid repeating shortcode outputs use a static variable in the shortcode handler:

function shortode_handler()
{
    static $done = FALSE;

    if ( $done )
        return;

    $done = TRUE;

    // create the shortcode output; this will happen only once.

    return $output;
}