<?php
function is_in_category($category_id) {
// if $category_id is not an array, create one
if (!is_array($category_id)) {
$category_id = array($category_id);
}
foreach (get_the_category() as $category) {
if (in_array($category->cat_ID, $category_id)) {
$is_in_category = true;
return true;
}
foreach (get_ancestors($category->cat_ID, 'category') as $ancestor) {
if ((in_array($ancestor, $category_id))) {
$is_in_category = true;
return true;
}
}
}
return false;
}