if( ! function_exists( 'videotube_child_add_rel_custom_page' ) ){

    /**
     * Add the rel meta tag into header.
     */

    function videotube_child_add_rel_custom_page() {

        global $post;

        if( ! is_singular() || ! isset( $post->post_content ) ){
            return;
        }

        if( defined( 'PHP_VERSION' ) && version_compare( PHP_VERSION , '5.3.0', '<' ) ){
            return;
        }

        if( has_shortcode( $post->post_content , 'videotube' ) && strpos( $post->post_content , 'navigation="on"') ){

            $max_num_pages  =   (int)get_post_meta( $post->ID, 'custom_max_num_pages', true );

            if( $max_num_pages == 0 ){
                return;
            }

            add_filter( 'wpseo_canonical' , '__return_null' );

            $page_var = is_front_page() ? 'page' : 'paged';

            $paged = get_query_var( $page_var ) ? get_query_var( $page_var ) : 2;

            if( ! is_paged() ){
                // add the next tag
                add_action( 'wpseo_head' , function() use( $paged, $max_num_pages ){
                    echo '<link rel="canonical" href="'. esc_url( get_permalink( $post->ID ) ) .'">';
                    if( $max_num_pages > 1 ){
                        echo '<link rel="next" href="'. esc_url( add_query_arg( array( 'paged' => $paged ), get_permalink( $post->ID ) ) ) .'">';
                    }

                });
            }
            else{
                add_action( 'wpseo_head' , function() use( $paged, $max_num_pages ){

                    echo '<link rel="canonical" href="'. esc_url( add_query_arg( array( 'paged' => $paged ), get_permalink( $post->ID ) ) ) .'">';
                    echo '<link rel="prev" href="'. esc_url( add_query_arg( array( 'paged' => $paged - 1 ), get_permalink( $post->ID ) ) ) .'">';

                    if( $max_num_pages >= ( $paged + 1 ) ){
                        echo '<link rel="next" href="'. esc_url( add_query_arg( array( 'paged' => $paged + 1 ), get_permalink( $post->ID ) ) ) .'">';
                    }

                });
            }

        }
    }

    add_action( 'wp', 'videotube_child_add_rel_custom_page', 21 );
}