WordPress: Function to check if current page is a direct child to another page

wp-child-page-conditional.php

<?php

/*
 * Check if the current pae is a direct child of another page.
 * Accept's page ID, page slug or page title as parameters to identify the parent.
 */

/* Add the following code in the theme's functions.php */ 
function is_child( $parent = '' ) {
  global $post;
  
  $parent_obj = get_page( $post->post_parent, ARRAY_A );
  $parent = (string) $parent;
  $parent_array = (array) $parent;
  
  if ( in_array( (string) $parent_obj['ID'], $parent_array ) ) {
    return true;
  } elseif ( in_array( (string) $parent_obj['post_title'], $parent_array ) ) {
    return true;    
  } elseif ( in_array( (string) $parent_obj['post_name'], $parent_array ) ) {
    return true;
  } else {
    return false;
  }
}

/* Usage:
  - with parent page ID:     is_child( 101 )
  - with parent page title:  is_child( 'About Us' )
  - with parent page slug:   is_child( 'about-us' )
  
 * Example:
  if ( is_child( 'About Us' ) ) {
    echo "This page is a direct child of About Us page";
  }
 */
?>