Using composer, add the required libraries. For generating a QR code for a text in drupal 8, follow the steps below.

1. Install the phpqrcode library using composer.

   composer require aferrandini/phpqrcode

2. In your custom module, create a php file, inside the 'src' folder (example: QrCodeGenerate.php) for handling the QRcode generation.
3. Create a class say QrCodeGenerate. Place the following code in the class 'QrCodeGenerate'.

   
         public function generateQrCodes($qr_text) {
         // The below code will automatically create the path for the img.
         $path = '';
         $directory = "public://Images/QrCodes/";
         file_prepare_directory($directory, FILE_MODIFY_PERMISSIONS | FILE_CREATE_DIRECTORY);
         // Name of the generated image.
         $qrName = 'myQrcode'.
         $uri = $directory . $qrName . '.png'; // Generates a png image.
         $path = drupal_realpath($uri);
         // Generate QR code image.
         \PHPQRCode\QRcode::png($qr_text, $path, 'L', 4, 2);
         return $path;
       }
       

4. Add use \Drupal\file\Entity\File; in your php file.

Now you can use the function generateQrCodes() to generate a QR code image from anywhere in your project. Hope that helps.