'responsive slider' core function v3

/**
 * Output the slider.
 *
 * @since 0.1
 */
function responsive_slider() {

$slide_return = '';

    $slides = new WP_Query( array( 'post_type' => 'slides', 'order' => 'ASC', 'orderby' => 'menu_order', 'posts_per_page' => -1 ) );

    if ( $slides->have_posts() ) :

        $slide_return .= '<div class="responsive-slider flexslider">

            <ul class="slides">';

            while ( $slides->have_posts() ) : $slides->the_post();

                $slide_return .= '<li>

                    <div id="slide-' . get_the_ID() . '" class="slide">';

                         global $post;

                             if ( has_post_thumbnail() ) :
                                $slide_return .= '<a href="' . get_post_meta( $post->ID, "_slide_link_url", true ) . '" title="' . the_title_attribute('echo=0') . '" >';
                                    $slide_return .= get_the_post_thumbnail( $post->ID, 'slide-thumbnail', array( 'class'   => 'slide-thumbnail' ) );
                                $slide_return .= '</a>';
                             endif;

                        $slide_return .= '<h2 class="slide-title"><a href="' . get_post_meta( $post->ID, "_slide_link_url", true ) . '" title="' . the_title_attribute('echo=0') . '" >' . get_the_title() .'</a></h2>';

                    $slide_return .= '</div><!-- #slide-x -->';

                $slide_return .= '</li>';

             endwhile;

            $slide_return .= '</ul>

        </div><!-- #featured-content -->';

     endif;

    wp_reset_query();

return $slide_return;
}