Bug Report Checklist

Description

Given a get request with optional, exploded, form-style, schema-type-array query parameters of type string (see OpenAPI declaration and steps-to-reproduce below).
When the generated php client code processes the array of query parameters.
Then it does not create the same result of ampersand-connected query as http://editor.swagger.io/ shows

Actual ?someParamName=0%3DoneValue%261%3DotherValue%262%3DanotherValue
Expected ?someParamName=oneValue&someParamName=otherValue&someParamName=anotherValue

openapi-generator version

master branch (version 3.3.4)

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: Example API
  version: 0.0.1
servers:
  - url: 'https://some.example.api.com/v1'
paths:
  /notes:
    get:
      summary: Returns all notes.
      operationId: fetchNotes
      parameters:
        - in: query
          name: labelIds
          required: false
          schema:
            type: array
            items:
              type: string
          style: form
          explode: true
      responses:
        '204':
          description: An object containing a list of group objects.

from example repo.

Eventually expect this ?labelIds=LabelOne&labelIds=LabelTwo&labelIds=LabelThree but the result is ?labelIds=0%3DLabelOne%261%3DLabelTwo%262%3DLabelThree.

Command line used for generation

openapi-generator generate -g php

Steps to reproduce

From https://github.com/githubERIK/openapi-php-multiparam-bug-example/commits/master

• Generate PHP client based on example.yaml
• Set name in composer.json
• Print failed query

... which is done in https://github.com/githubERIK/openapi-php-multiparam-bug-example.

After that clone try it out like in https://github.com/githubERIK/openapi-php-multiparam-bug-tryout with steps described in the README.

Related issues/PRs

Similar, but different: #1763.

Suggest a fix

Workaround: in the generated file under the comment // query params: lib/Api/DefaultApi.php#L234 replace

this

        if (is_array($label_ids)) {
            $label_ids = ObjectSerializer::serializeCollection($label_ids, 'multi', true);
        }
        if ($label_ids !== null) {
            $queryParams['labelIds'] = ObjectSerializer::toQueryValue($label_ids);
        }

by this

$queryParams['labelIds'] = $label_ids;