Testing for sub-Pages

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

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;
    }
}

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.

/**
 * 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;
    }
}