<?php if(is_category()): ?>
<?php
$categoryname = single_cat_title('',false);
$categoryid = get_cat_ID($categoryname);
?>
<?php elseif(is_single()) : ?>
<?php
$post_cat = get_the_category();
$categoryid=$post_cat[0]->cat_ID;
$categoryname = $post_cat[0]->cat_name;
?>
<?php endif; ?>