WordPress: Check if a page is child of another

is_child.php

/**
 * Check if a page is child of another 
 * Ref: http://bavotasan.com/2011/is_child-conditional-function-for-wordpress/
 * 
 * @param mixed $page_id_or_slug Provide the parent ID or Slug
 * @return bool Whether the page is child or not of the given parent
 */
function is_child( $page_id_or_slug ) 
{ 
    global $post; 
    if( !is_page() )
        return false;

    # Slug provided, get ID
    if( !is_int( $page_id_or_slug ) ) 
    {
        $page = get_page_by_path( $page_id_or_slug );
        $page_id_or_slug = $page->ID;
    } 

    return $post->post_parent == $page_id_or_slug; 
}