PHP 7.x compatibility - #82

        $contentTypes = array(
            'csv'  => 'text/csv; charset=' . $charset,
            'xlsx' => 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
            'ods'  => 'application/vnd.oasis.opendocument.spreadsheet',
        );

        $writerClasses = array(
            'csv'  => Csv::class,
            'xlsx' => Xlsx::class,
            'ods'  => Ods::class,
        );

        if (!isset($contentTypes[$getMode], $writerClasses[$getMode])) {
            throw new InvalidArgumentException('Invalid mode');
        }

        $contentType = $contentTypes[$getMode];
        $writerClass = $writerClasses[$getMode];