How to check if page is parent or child or grandchild of a page

check-page-parent-child-grandchild-page.php

<?php

// Check for a specific page
if ( is_page(2) ) {
  // stuff
}

// Check if page is child of a certain page
if ( $post->post_parent == '2' ) {
  // stuff
}

// Is page a parent or child of the parent
function is_tree($pid) {      // $pid = The ID of the page we're looking for pages underneath
	global $post;         // load details about this page
	if(is_page()&&($post->post_parent==$pid||is_page($pid))) 
               return true;   // we're at the page or at a sub page
	else 
               return false;  // we're elsewhere
};

// Is page a parent, child or any ancestor of the page
function is_tree($pid)
{
  global $post;

  $ancestors = get_post_ancestors($post->$pid);
  $root = count($ancestors) - 1;
  $parent = $ancestors[$root];

  if(is_page() && (is_page($pid) || $post->post_parent == $pid || in_array($pid, $ancestors)))
  {
    return true;
  }
  else
  {
    return false;
  }
};

// Usage

if (is_tree(2)) {
   // stuff
}