/** Validates 'PHP_AUTH_USER' and 'PHP_AUTH_PW'
	 * @return string error message or null if valid
	 * Default implementation is for single byte character encodings, 
	 * all characters are expected to be valid (like with ISO-8859-1).
	 * Should be overridden for UTF-8 and other character encodings for which 
	 * invalid characters can occur.
	 */
	function validatePhpAuth($name, $value) {
		return $this->validateForNullChar($name, $value);
	}