public function GetVDataString
	(
		$nErrorId,
		$sErrorDesc	= '',
		$arrVData	= [],
		$sVersion	= '1.0',
		$bCached	= null,
		$arrExtra	= []
	)
	{
		//
		//	nErrorId	- [in] int		error id
		//	sErrorDesc	- [in/opt] string	error description
		//	arrVData	- [in/opt] array	virtual data
		//	sVersion	- [in/opt] string	service version, default is '1.0'
		//	bCached		- [in/opt] bool		if the data come from cache
		//	arrExtra	- [in/opt] array	extra data by key-value pairs
		//	RETURN		- encoded json string
		//
		$sRet = '';

		//	...
		$arrJson = $this->GetVDataArray( $nErrorId, $sErrorDesc, $arrVData, $sVersion, $bCached, $arrExtra );
		if ( $this->IsValidVData( $arrJson ) )
		{
			$sRet = @ json_encode( $arrJson );
		}
		else
		{
			$sRet = @ json_encode( $this->GetDefaultVDataJson() );
		}

		return $sRet;
	}