Changes a user's display name on PMPro Member Directory and Member Profile pages to their login name.

my-pmpro-change-display-name-member-directory.php

<?php
/**
 * This recipe changes a user's display name on directory and
 * profile pages with their login name.
 *
 * This requires the latest version (1.0+) of
 * the Member Directory and Profile Pages Add On
 * https://www.paidmembershipspro.com/add-ons/member-directory/
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_member_directory_change_display_name( $display_name, $user ) {
	$display_name = $user->user_login;
	return $display_name;
}
add_filter( 'pmpro_member_directory_display_name', 'my_pmpro_member_directory_change_display_name', 10, 2 );