chamilo-lms2/main/inc/lib/formvalidator/Rule/UsernameAvailable.php

<?php
/* For licensing terms, see /license.txt */

require_once 'HTML/QuickForm/Rule.php';
/**
 * QuickForm rule to check if a username is available
 */
class HTML_QuickForm_Rule_UsernameAvailable extends HTML_QuickForm_Rule
{
	/**
	 * Function to check if a username is available
	 * @see HTML_QuickForm_Rule
	 * @param string $username Wanted username
	 * @param string $current_username
	 * @return boolean True if username is available
	 */
	function validate($username, $current_username = null) {
		$user_table = Database::get_main_table(TABLE_MAIN_USER);
        $username = Database::escape_string($username);
        $current_username = Database::escape_string($current_username);

		$sql = "SELECT * FROM $user_table WHERE username = '$username'";
		if (!is_null($current_username)) {
			$sql .= " AND username != '$current_username'";
		}
		$res = Database::query($sql);
		$number = Database::num_rows($res);
		return $number == 0;
	}
}