/**
 * Implementation of hook_user().
 */
function realname_registration_user($op, &$edit, &$account, $category = NULL) {
  if ($op == 'insert') {
    // Generate the username based on first name and last name fields.
    $first_init = drupal_substr($edit['firstname'], 0, 1);
    $lastname = $edit['lastname'];
    $username = strtolower($first_init . $lastname);
    $orig = $username;
    $i = 0;

    // Check if the username already exists in the database.
    while (db_result(db_query("SELECT COUNT(*) FROM {users} WHERE name = '%s';", $username))) {
      $username = $orig . ++$i;
    }

    $edit['name'] = $username;
  }
}