I just want to save the file

[ftrotter]

I do not want to display or output anything.
I am sure there is a way, but I cannot easily see it in the documentation or the examples.

-FT

[zzplab]

chillerlan/php-qrcode#advanced-usage

// ...with additional cache file
$qrcode->render($data, '/path/to/file.png');

[codemasher]

See #19.
Since v3.0 you can also issue a cache file path via QRCode::render(string $data, string $file = null).

edit: thank you @zzplab!

edit2:
Note that the file extension/type won't be checked. You need to make sure that the output type matches the file extension.