[BUG] [PHP] Enum values starting with - have the same name as ones without, resolving in duplicate consts

[victorlap]

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?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Currently enum values that are the same except for a starting - result in the same const names for php model files. This result in unexecutable files, that need manual editing before it can be used.

openapi-generator version

both on docker:
openapi-generator-cli 5.0.0-SNAPSHOT
commit : a28037d
built : 2020-09-01T09:02:56Z
openapi-generator-cli 5.0.0-beta2
commit : 212b4f7
built : 2020-09-04T05:08:30Z

OpenAPI declaration file content or url

https://gist.github.com/victorlap/44e351b72f5d9f26f0ac1943b4c6cf22

Command line used for generation

docker run --rm -v ${PWD}:/local openapitools/openapi-generator-cli:v5.0.0-beta2 generate \
    -i /local/spec.json \
    -g php \
    -o /local/src

Steps to reproduce

Run the above command on the spec that is linked in the gist.
Check file src/lib/Model/SalesOrderBillingaddress.php and see duplicate const declaration:

    const CUSTOM_FORM__1049 = '-1049';
    const CUSTOM_FORM__1049 = '1049';

Related issues/PRs

I've not found any related issues/PRs

Suggest a fix/enhancement

I think the sanitizing of the name needs to change a bit.

It's probably somewhere in the toEnumVarName