Creating username with firstname and lastname (first letter of firstname + lastname)

username.php

<?php
function parse_login($start, $firstname, $lastname) {
  $prefix = substr($firstname, 0, $start + 1);
  $login = substr($prefix . $lastname, 0, 10);
  return strtolower($login);
}

function define_login($mysql, $firstname, $lastname) {
  $idx = 0;
  $login = parse_login($idx, $firstname, $lastname);
  $suffix = 1;
  $prefixIdx = 1;
  // countUserWithUsername is a function which count the user with this
  // specific username. You have to do it yourself, according to the specificities
  // of your tables
  while (countUserWithUsername($mysql, $login) > 0) {
      if ($idx >= strlen($firstname)) {
          if (strlen($firstname) >= $prefixIdx - 1) {
              $prefixIdx = 1;
              $suffix = 1;
          }
          $login = parse_login($prefixIdx - 1, $firstname, $lastname) . $suffix;
          $prefixIdx++;
      } else
          $login = parse_login($idx++, $firstname, $lastname);
  }
  return ($login);
}