title: Why get_posts from a category id doesn't work
body: <p>use the following code to get portfolio type posts from category id, but it doesn't work.</p>
  
  <pre><code>$args = array( 'numberposts' =&gt; $items, 'post_type'=&gt;'portfolio','orderby' =&gt; 'post_date','order'=&gt;'DESC','category' =&gt; 5 );
  $posts = get_posts($args);
  foreach($posts as $post){
      setup_postdata($post);
      $return_html='......';
  }
  </code></pre>
  
  <p>however,if I leave the category id empty, it' fine and will show all categories posts. I'm sure that there are many posts under category 5.</p>
  
  <p>Thanks in advance.</p>