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<?php
/**
 * Tests if any of a post's assigned categories are descendants of target categories
 *
 * @param int|array $cats The target categories. Integer ID or array of integer IDs
 * @param int|object $_post The post. Omit to test the current post in the Loop or main query
 * @return bool True if at least 1 of the post's categories is a descendant of any of the target categories
 * @see get_term_by() You can get a category by name or slug, then pass ID to this function
 * @uses get_term_children() Passes $cats
 * @uses in_category() Passes $_post (can be empty)
 * @version 2.7
 * @link http://codex.wordpress.org/Function_Reference/in_category#Testing_if_a_post_is_in_a_descendant_category
 */
if ( ! function_exists( 'post_is_in_descendant_category' ) ) {
  function post_is_in_descendant_category( $cats, $_post = null ) {
    foreach ( (array) $cats as $cat ) {
      // get_term_children() accepts integer ID only
      $descendants = get_term_children( (int) $cat, 'category' );
      if ( $descendants && in_category( $descendants, $_post ) )
        return true;
    }
    return false;
  }
}
?>