post - wordpress is_category of all subcategories of parent category -

i want show certien code inside posts (single.php) assosiated sub cats of 1 father cat. want code work post under sub cats (that under main cat)

i have tryed this:

<?php function get_child_cats( $catname ) {     $parentcat = get_cat_id(8);     $subcat = get_categories( array('child_of' => $parentcat ) );     $cat_array = array();     array_push($cat_array, $parentcat); // add parent cat array     foreach ($subcat $sc) {         array_push($cat_array, $sc->cat_id);     }     return $cat_array; }?> 

and this:

<?php  if (in_category(8) && !is_feed()) { ?> 

this how done:

<?php $mycats = array(8);     foreach (get_the_category() $childcat) {         foreach ($mycats $mycat) {             if (cat_is_ancestor_of($mycat, $childcat)) { ?> <ul> <?php wp_list_categories('orderby=id&show_count=1&use_desc_for_title=0&child_of=8'); ?></ul>              <?php   break 2;             }         }     } ?>