Shortcode for related posts

[ Moved to the Fixing WordPress sub-forum. ]

Hello,
I’m using bellow function for related post shortcode and I have two question about it, first: it’s supposed to display posts with related tag but it doesn’t do that what’s wrong?. Second: how the code should be changed so I can use something like [rps tag=google] and the function just return a post has a tag as “google”?
This is the code:

add_shortcode('rps', 'fphp_get_related_posts');
function fphp_get_related_posts() {
    $reset_post = $post;
    global $post;
    $post_tags = wp_get_post_tags($post->ID);

    if ($post_tags) {
        $post_tag_ids = array();
    foreach($post_tags as $post_tag) $post_tag_ids[] = $post_tag->term_id;
        $args=array(
            'tag__in' => $post_tag_ids,
            'post__not_in' => array($post->ID),
            'posts_per_page' => 1,
      		'orderby' => 'rand',
        );

    $related_query = new wp_query( $args );
    if (intval($related_query->post_count) === 0) return '';

    $html = '<div class="rps"><ul><h3>همچنین بخوانید</h3>';
      $related_query->the_post();
      $html .= '<li style="width:250px">';
      $html .= '<div class="relatedthumb"><a rel="external" href="'. get_the_permalink(). '">';
      $html .= get_the_title() . '</a>';
      $html .= '</div></li>';
  }
  $post = $reset_post;
  wp_reset_query();

  $html .= '</ul></div>';
  return $html;
}