Convert capital letters to lowercase and replace spaces with underscores "_" in order to standardize usernames.

replace-spaces-with-underscores.php

<?php // Do not copy this tag.
/**
 * Convert capital letters to lowercase and replace all spaces with an underscore when 
 * new users register for Paid Memberships Pro.
 * Add this code to your PMPro Customizations Plugin -  
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
 
 function my_custom_user_registration_changes( $userdata ) {

	$userdata['user_login'] = str_replace(' ', '_', strtolower( $userdata['user_login'] ) );

	return $userdata;
}
add_filter( 'pmpro_checkout_new_user_array', 'my_custom_user_registration_changes', 10, 1 );

This recipe is included in the blog post "Reformat usernames to use allowed characters after checkout." at Paid Memberships Pro here: https://www.paidmembershipspro.com/reformat-usernames-to-use-allowed-characters-after-checkout/