事实证明，在Wordpress forums is_tree()

function is_tree($pid) {      // $pid = The ID of the page we\'re looking for pages underneath
    global $post;         // load details about this page

    $anc = get_post_ancestors( $post->ID );
    foreach($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
};

<?php if(is_tree(12)){echo \'foobar\';} ?>