In looking at the source for guzzlehttp/guzzle 5.3 (https://github.com/guzzle/guzzle/tree/5.3/src), there is no Middleware class, so it's no surprise that I'm getting a "Class does not exist" exception.
This can be resolved by either dropping support for 5.3 in your composer.json file, or refactoring your usage of the Middleware class to be compliant with 5.3 and 6.
Also, installation should have been blocked by incompatible Guzzle versions, but it was not because your guzzlehttp/guzzle requirement is inside require-dev instead of require in your composer.json file.
In looking at the source for guzzlehttp/guzzle 5.3 (https://github.com/guzzle/guzzle/tree/5.3/src), there is no Middleware class, so it's no surprise that I'm getting a "Class does not exist" exception.
This can be resolved by either dropping support for 5.3 in your
composer.json
file, or refactoring your usage of theMiddleware
class to be compliant with 5.3 and 6.