Method getInstance

/**
 * Returns the global Browser object, only creating it
 * if it doesn't already exist.
 *
 * @param   string  $userAgent  The browser string to parse.
 * @param   string  $accept     The HTTP_ACCEPT settings to use.
 *
 * @return  Browser  The Browser object.
 *
 * @since   1.7.0
 */
public static function getInstance($userAgent = null, $accept = null)
{
    $signature = serialize(array($userAgent, $accept));
    if (empty(self::$instances[$signature])) {
        self::$instances[$signature] = new static($userAgent, $accept);
    }
    return self::$instances[$signature];
}