A possible response format for APIs is in a PHP serialized string. This isn't good for many reasons:

• readable only by PHP
• prone to not work across systems or across time (e.g. differences between HHVM and the zend engine)
• very fragile (e.g. some plugin replace words in responses, this break this format)
• not standard at all (JSON, XML & co are more than enough for an HTTP API)

We could deprecate it for 3.0 and remove it later.