Custom usernames with numeric suffix if the username exists

my_pmpro_custom_username.php

<?php
/**
 * Build usernames from first name and last name 
 * Add numeric suffix if username exists
 *
 * Add this code to your PMPro Customizations Plugin -  
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
 
 function my_pmpro_custom_username( $userdata ) {

	if ( ! empty( $_REQUEST['first_name'] ) && ! empty( $_REQUEST['last_name'] ) ) {

		$first_name = ucfirst( strtolower( $_REQUEST['first_name'] ) );
	    $last_name  = ucfirst( strtolower( $_REQUEST['last_name'] ) );

	    $user_login = $first_name . $last_name;

		// Check if username exists and add/increment suffix
		$new_user_login = $user_login;

		do {
			$user_login_exists = get_user_by( 'login', $new_user_login );
			if($user_login_exists) {
				$count = intval( filter_var( $new_user_login, FILTER_SANITIZE_NUMBER_INT ) ); 
				$count++;
				$new_user_login = $user_login . $count;
			}
		}  while( $user_login_exists );

		$userdata['user_login'] = $new_user_login;

	}

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

/**
 * Make the username field not required on checkout.
 */
function my_pmpro_unset_required_username_checkout( $pmpro_required_user_fields ) {
	unset( $pmpro_required_user_fields['username'] );
	return $pmpro_required_user_fields;
}
add_filter('pmpro_required_user_fields', 'my_pmpro_unset_required_username_checkout', 10, 2);