Filter not getting triggered

It’s not working. Located in functions.php:

add_filter('ghpseo_output', 'my_seo_output', 10, 2); 

function my_seo_output($output='', $type='main') {

	echo "hello";

    switch ($type) {
        //case "main": $result = $output; break;
        //case "main_title": $result = $output; break;
        //case "secondary_title": $result = $output; break;
        case "description": if( is_single() ) {
								$standfirst = get_post_meta($wp_query->post->ID,'subtitle',true);
								$result = '<meta name="description" content="' . $standfirst . '" />';

							} else {
								$result = $output;
							}
							break;
        default: $result = $output;
        }
    return $result;
}