Generate a unique username in WordPress

generate-unique-username.php

<?php
/**
 * Recursive function to generate a unique username.
 *
 * If the username already exists, will add a numerical suffix which will increase until a unique username is found.
 *
 * @param string $username
 *
 * @return string The unique username.
 */
function generate_unique_username( $username ) {

	static $i;
	if ( null === $i ) {
		$i = 1;
	} else {
		$i++;
	}

	if ( ! username_exists( $username ) ) {
		return $username;
	}

	$new_username = sprintf( '%s-%s', $username, $i );

	if ( ! username_exists( $new_username ) ) {
		return $new_username;
	} else {
		return call_user_func( __FUNCTION__, $username );
	}
}