Entity.TestObject.yml

MyCompany\MyProject\CoreBundle\Entity\TestObject:
    exclusion_policy: ALL
    properties:
        id:
            expose: true
            type: integer
        string:
            expose: true
        createdAt:
            access_type: public_method
            accessor:
                getter: getCreatedAt
                setter: setCreatedAt
            expose: true
            type: DateTime

result.json

{
    "id": 23,
    "string": "testing..."
}

TestObject.php

<?php

namespace MyCompany\MyProject\CoreBundle\Entity;


class TestObject
{
    protected $id = 23;

    protected $string = 'testing...';

    protected $createdAt;

    public function __construct()
    {
        return new \DateTime;
    }

    public function getCreatedAt()
    {
        return $this->createdAt;
    }

    public function setCreatedAt(\DateTime $createdAt)
    {
        $this->createdAt = $createdAt;
    }

    public function getId()
    {
        return $this->id;
    }

    public function getString()
    {
        return $this->string;
    }
}