Testing for sub-Pages

There is no is_subpage() function yet, but you can test this with a little code:

Snippet 1

// If outside the loop.
$post = get_post();

if ( is_page() && $post->post_parent ) {
    // This is a subpage
} else {
    // This is not a subpage
}

You can create your own is_subpage() function using the code in Snippet 2. Add it to your functions.php file. It tests for a parent page in the same way as Snippet 1, but will return the ID of the page parent if there is one, or false if there isn’t.

Snippet 2

/**
 * Check whether we are on a subpage
 *
 * @return mixed ID of the parent post or false if this is not a subpage.
 */
function wpdocs_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 ) {
		// Return 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;
	}
}

It is advisable to use a function like that in Snippet 2, rather than using the simple test like Snippet 1, if you plan to test for sub pages frequently.

To test if the parent of a page is a specific page, for instance “About” (page id pid 2 by default), we can use the tests in Snippet 3. These tests check to see if we are looking at the page in question, as well as if we are looking at any child pages. This is useful for setting variables specific to different sections of a web site, so a different banner image, or a different heading.

Snippet 3

<?php
if ( is_page( 'about' ) || '2' == $post->post_parent ) {    
	// the page is "About", or the parent of the page is "About"
	$bannerimg = 'about.jpg';
} elseif ( is_page( 'learning' ) || '56' == $post->post_parent ) {	
	$bannerimg = 'teaching.jpg';
} elseif ( is_page( 'admissions' ) || '15' == $post->post_parent ) { 
	$bannerimg = 'admissions.jpg';
} else { 
	$bannerimg = 'home.jpg'; // just in case we are at an unclassified page, perhaps the home page
}	
?>

Snippet 4 is a function that allows you to carry out the tests above more easily. This function will return true if we are looking at the page in question (so “About”) or one of its sub pages (so a page with a parent with ID “2”).

Snippet 4

/**
 * 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 wpdocs_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
}

Add Snippet 4 to your functions.php file, and call is_tree( 'id' ) to see if the current page is the page, or is a sub page of the page. In Snippet 3, is_tree( '2' ) would replace “is_page( 'about' ) || '2' == $post->post_parent” inside the first if tag.

Note that if you have more than one level of pages the parent page is the one directly above and not the one at the very top of the hierarchy.