Force to have same display name as username in wordpress

Force to have same display name as username in wordpress.php

<?php

add_action('init', 'change_display_name' );
add_action( 'profile_update', 'change_display_name' );
function change_display_name() {
	$users = get_users();
	foreach( $users as $user ){
		if( $user->user_login != $user->display_name ){
			wp_update_user( array ('ID' => $user->ID, 'display_name'=> $user->user_login) ) ;
		}
	}
}