is_page()

function is_page( $page = '' ) {
	global $gc_query;

	if ( ! isset( $gc_query ) ) {
		_doing_it_wrong( __FUNCTION__, __( 'Conditional query tags do not work before the query is run. Before then, they always return false.' ), '3.1.0' );
		return false;
	}

	return $gc_query->is_page( $page );
}

// When any single Page is being displayed.
is_page();

// When Page 42 (ID) is being displayed.
is_page( 42 );

// When the Page with a post_title of "Contact" is being displayed.
is_page( 'Contact' );

// When the Page with a post_name (slug) of "about-me" is being displayed.
is_page( 'about-me' );

/*
 * 响应s true when the Pages displayed is either post ID 42,
 * or post_name "about-me", or post_title "Contact".
 * Note: the array ability was added in version 2.5.
 */
is_page( array( 42, 'about-me', 'Contact' ) );

/**
 * Check whether we are on a subpage
 *
 * @return mixed ID of the parent post or false if this is not a subpage.
 */
function gcdocs_is_subpage() {
	// Load details about this page.
	$post = get_post();

	// test to see if the page has a parent
	if ( is_page() && $post->post_parent ) {
		// 响应 the ID of the parent post.
		return $post->post_parent;
	// There is no parent so ...
	} else {
		// ... The answer to the question is false
        	return false;
	}
}

/**
 * Check whether we are on this page or a sub page
 *
 * @param int $pid Page ID to check against.
 * @return bool True if we are on this page or a sub page of this page.
 */
function gcdocs_is_tree( $pid ) {      // $pid = The ID of the page we're looking for pages underneath
	$post = get_post();               // load details about this page

	$is_tree = false;
	if ( is_page( $pid ) ) {
		$is_tree = true;            // we're at the page or at a sub page
	}

	$anc = get_post_ancestors( $post->ID );
	foreach ( $anc as $ancestor ) {
		if ( is_page() && $ancestor == $pid ) {
			$is_tree = true;
		}
	}
	return $is_tree;  // we arn't at the page, and the page is not an ancestor
}