// conversion from JSON to PHP
application/json => application/x-php | {"productId": 1} => new OrderProduct(1)
// conversion from PHP to JSON
application-x-php => application/json | new OrderProduct(1) => {"productId": 1}
// conversion from XML to PHP
application/xml => application/x-php | <productId>1</productId> => new OrderProduct(1)
// conversion from PHP to XML
application-x-php => application/xml | new OrderProduct(1) => <productId>1</productId>
// conversion from JSON to PHP Array
application/json => application/x-php;type=array | {"productId": 1} => ["productId": 1]
// conversion from PHP Array to JSON
application/x-php;type=array => application/json | {"productId": 1} => ["productId": 1]
// conversion from XML to PHP Array
application/xml => application/x-php;type=array | <productId>1</productId> => ["productId": 1]
// conversion from PHP Array to XML
application/x-php;type=array => application/xml | ["productId": 1] => <productId>1</productId>