[BUG] [PHP] Some models do not have a baseType

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

Some models do not have a baseType property, which causes broken links (e.g. ../Model/.md) and duplicated qualifiers (e.g. \OpenAPI\Client\Model\\OpenAPI\Client\Model\StringEnumRef) in the documentation of the PHP generator.

openapi-generator version

This issue occurs on master (7d8eacc197656fbf4360ccb6dc0ffb24bc0219d4)

OpenAPI declaration file content or url

The issue occurs with the echo_api spec included in the generator samples.

Generation Details

See the php-nextgen-echo-api.yaml config

Steps to reproduce

Related issues/PRs

This issue was previously mentioned in #19567

Suggest a fix

Finding the reason the baseType is not set and solving it should fix this issue.

Start with the echo_api spec and the php-nextgen-echo-api.yaml config, then reproduce the generated PHP documentation. Compare models without a baseType to the broken links and duplicated qualifiers described here, and review issue #19567 for context. Done means the affected generated documentation no longer contains those malformed references.