Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/IOFactory.php
public static function registerReader($readerType, $readerClass)
    {
        if (!is_a($readerClass, Reader\IReader::class, true)) {
            throw new Reader\Exception('Registered readers must implement ' . Reader\IReader::class);
        }

        self::$readers[$readerType] = $readerClass;
    }
public static function registerReader($readerType, $readerClass)
    {
        if (!is_a($readerClass, Reader\IReader::class, true)) {
            throw new Reader\Exception('Registered readers must implement ' . Reader\IReader::class);
        }

        self::$readers[$readerType] = $readerClass;
    }
Register a reader with its type and class name.

@param string $readerType
@param string $readerClass