Wordpress 親カテゴリのIDを取得

category.php

$cateid = get_query_var('cat');//現在のカテゴリID
$child = get_category($cateid);
$parentID = $child->parent; //これも親ID
$parentID = $child->category_parent;//これでも親ID

single.php

$categories = get_the_category();
foreach ( $categories as $category ) {
   $cat_id = $category->cat_ID;
   $child = get_category($cat_id);
   $parentID = $child->parent;
}

親カテゴリの名前を取得

single.php

$categories = get_the_category();
foreach ( $categories as $category ) {
   $cat_name = $category->name;
   $catenameParent = $cat_name;
}