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.
 * 
 * REF: https://gist.github.com/philipnewcomer/59a695415f5f9a2dd851deda42d0552f
 *
 * @param string $username
 *
 * @return string The unique username.
 */
function generate_unique_username($username)
{
    $username = sanitize_user($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);

    }

}