/** In case invalid character encoding is possible, this method shoud be overridden to validate the character encodings of $value.
	 * @param string $name properly encoded in $this->gpcCharset, or 'param name'
	 * @param string $value to be checked to be valid for $this->gpcCharset
	 * @return string validation error message or null if valid
	 */ 
	function validateGpcValue($name, $value) {
		return session_name() === $name
			? $this->validateSessionId($name, $value) 
			: $this->validateForNullChar($name, $value);
	}