[BUG] PHP Generator omits Namespace of Models in $allOf

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

The PHP Generator seems to omit the class namespace for components referenced within allOf, leading to a class not found error during deserialization. I created a repository showing the generated code https://github.com/rcky/openapi-generator-all-of .

Fatal error: Uncaught Error: Class 'TestDto' not found in .../openapi-generator-all-of/lib/ObjectSerializer.php:366

openapi-generator version

Tested with 4.3.1, 5.0.0, 5.0.1

OpenAPI declaration file content or url

openapi.yml

openapi: '3.0.1'
info:
  title: Test allOf PHP
  version: '1.0'
paths:
  /test:
    get:
      tags: [ Test ]
      responses:
        default:
          description: Say hi
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/CompoundDto'
components:
  schemas:
    CompoundDto:
      title: CompoundDto
      type: object
      properties:
        testProperty:
          description: Property using allOf
          allOf:
            - $ref: '#/components/schemas/TestDto'
    TestDto:
      title: TestDto
      type: object
      properties:
        name:
          type: string

Generation Details

Generated it using the docker image. generate -i /local/openapi/openapi.yml -g php -o /local. Find more details in the corresponding docker-compose.yml

Steps to reproduce

One can already see it in the corresponding model definitions. If someone wants to test the corresponding code:

• Checkout the corresponding repos https://github.com/rcky/openapi-generator-all-of
• Dump the autoloader composer dump-autoloader
• Execute the tryout.php file (php tryout.php).

Start with openapi.yml and the generated lib/ObjectSerializer.php, then reproduce the failure using docker-compose.yml and tryout.php. Compare the allOf TestDto reference in the generated model definitions with the deserialization error at ObjectSerializer.php:366; done means the generated PHP code resolves the model namespace and tryout.php completes without a class-not-found error.