[PHP][Symfony] Further enhancements

[naelrashdeen]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
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Description of the PR

The current PHP Symfony needs some further changes, which are described below:

• Headers are not being fetched properly in the request
• Objects should be mapped to arrays instead, in the model classes to be compatible with Symfony type validation
• Deserialising arrays as collection formats are not working properly
• Correct data type is not set in the comments for the Api interfaces

Closes #6999

[wing328]

cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh