[BUG] [PHP] Object of class stdClass could not be converted to string

Bug Report Checklist

Description

generating the php classes of these open-api definitions:
https://developer.ebay.com/api-docs/sell/analytics/overview.html

leads to a problem i have in ObjectSerializer.php
where i get the error:

Object of class stdClass could not be converted to string

openapi-generator version

tested: 3.3.4 (php5) and 4.3.1 (php7)

OpenAPI declaration file content or url

https://developer.ebay.com/api-docs/master/sell/analytics/openapi/3/sell_analytics_v1_oas3.yaml
https://developer.ebay.com/api-docs/master/sell/analytics/openapi/3/sell_analytics_v1_oas3.json

Command line used for generation

generate -i /app/sell_analytics_v1_oas3.yaml -g php -o /app/ --config /app/openapi-config.php.ebay.analytics.json

Steps to reproduce

testing this code:

php7: https://github.com/michabbb/sdk-ebay-rest-analytics
php5: https://github.com/michabbb/sdk-ebay-rest-analytics/tree/php5

for real testing, you need an eBay oauth code, then you can use the generated code like that:

$config = Configuration::getDefaultConfiguration()->setAccessToken($ebay_access_token);
        $apiInstance = new SellerStandardsProfileApi(null,$config);
        try {
            $result = $apiInstance->findSellerStandardsProfiles();
            +d($result);
        } catch (\Exception $e) {
            echo 'Exception when calling CustomerServiceMetricApi->getCustomerServiceMetric: ', $e->getMessage(), PHP_EOL;
        }

here you can see an example API result of findSellerStandardsProfiles

Suggest a fix

in ObjectSerializer.php line 299 where this code is in:

} elseif (in_array($class, ['DateTime', 'bool', 'boolean', 'byte', 'double', 'float', 'int', 'integer', 'mixed', 'number', 'object', 'string', 'void'], true)) {
            settype($data, $class);
            return $data;
        }

i have to change:

} elseif (in_array($class, ['DateTime', 'bool', 'boolean', 'byte', 'double', 'float', 'int', 'integer', 'mixed', 'number', 'object', 'string', 'void'], true)) {
            if ($data instanceof \stdClass) {
                settype($data,'array');
            } else {
                settype($data, $class);
            }
            return $data;
        }

that is a hack not a fix... i know... i don´t know whats wrong here.
thanks for any help!!!

Start at ObjectSerializer.php around line 299 and reproduce the failure with the linked eBay OpenAPI YAML or JSON using the provided generation command. Compare the generated SDK behavior with the example API result and determine the correct handling for stdClass values; done means the generated PHP client can process that result without the conversion error.