[BUG] [PHP] PSR-18 generator has the wrong argument to selectHeaders

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

Had this issue in a generated client and was able to find that it's present in the example client code for the PetStore API.

https://github.com/OpenAPITools/openapi-generator/blob/master/samples/client/petstore/php/psr-18/lib/HeaderSelector.php#L38

The 2nd argument to HeaderSelector is a string.

If you look at the usages on it you'll find for example, https://github.com/OpenAPITools/openapi-generator/blob/master/samples/client/petstore/php/psr-18/lib/Api/StoreApi.php#L340

showing that it tries to pass an array as the 2nd argument to selectHeaders. This fails when you try to call any API method, with:

PHP Fatal error: Uncaught TypeError: <your namespace>\HeaderSelector::selectHeaders(): Argument #2 ($contentType) must be of type string, array given

openapi-generator version

Latest master

OpenAPI declaration file content or url

As per the generated sample in the project

Generation Details

Steps to reproduce

Generate the sample PetStore client with the PSR-18 library.

Related issues/PRs

Suggest a fix

Start with samples/client/petstore/php/psr-18/lib/HeaderSelector.php and the selectHeaders call in samples/client/petstore/php/psr-18/lib/Api/StoreApi.php. Generate the PetStore client with the PSR-18 library and reproduce the PHP TypeError. Done means the generated client passes the correct argument type and API methods run without this error.