Check if the current page (or a given page) is in the child tree of a given page

This function checks if the current page (or a given page, second parameter) is in the child tree of a given page.

/* Tests, if a page has got a certain parent anywhere in the ancestor/children tree */
function is_tree($pid, $pid2check = 0) {// $pid = The ID of the page we're looking for pages underneath
 
	global $post;
	// load details about this page
 
	// check if we should check two posts independently from the current query
	if ($pid2check < 1) {
		$pid2check = $post -> ID;
	}
 
	$anc = get_post_ancestors($pid2check);
 
	foreach ($anc as $ancestor) {
		if (is_page() && $ancestor == $pid) {
			return true;
		}
	}
 
	if (is_page() && (is_page($pid))) {
		return true;
		// we're at the page or at a sub page
	} else {
		return false;
		// we're elsewhere
	}
}

Check if the current post (or a given post) is in the ancestor/child tree of a given category

This function checks if the current post (or a given post identified by ID, second parameter) is in the ancestor/child tree of a given category.

/* Tests, if the given category is the current category, or in the ancestor/child tree of a given post or a child of it */
function is_category_or_sub($cat_id = 0, $pid = 0) {
	global $post;
 
	// check if a custom post id was given or get default (standard)
	if($pid<1) {
		$pid=$post->ID;
	}
 
	// check it for each category
    foreach (get_the_category($pid) as $cat) {
    	if ($cat_id == $cat->cat_ID || cat_is_ancestor_of($cat_id, $cat))  {
    		return true;
        }
    }
    return false;
}

Check if a given category is in the current category tree

Helpful if you have to determine the current category manually – checks if a given category is in the tree of another given category.

function in_category_tree($parent_cat,$child_or_equal_cat) {
	if($parent_cat ==$child_or_equal_cat || cat_is_ancestor_of($parent_cat, $child_or_equal_cat)) {
		return true;
	}
	return false;
}

Custom Taxonomy hierarchy check

Determine the top level taxonomy or check if a taxonomy is in the current taxonomies hirarchical tree.

function is_taxonomy_or_sub($child_or_equal_term_id,$parent_term_id,$taxonomy_name) {
       if($parent_term_id==$child_or_equal_term_id || $parent_term_id == get_top_level_term($child_or_equal_term_id,$taxonomy_name))  {
                return true;
        }
 }
 
function get_top_level_term($term_id,$taxonomy_name) {
	$term = get_term_by('id',$term_id,$taxonomy_name);
	if($term->parent>0) {
		return get_top_level_term($term->parent,$taxonomy_name);
	} else {
		return $term->term_id;
	}
}

This article is still under construction. Please report bugs, errors or improvement ideas, so we can optimize these scripts.