example.php

<?php

$em = getYourEntityManager();
$object = $em->getRepository('Object')->find(1);

$serializer = new ObjectSerializer($em);
$serialized = $serializer->serialize($object, 2);

ObjectSerializer.php

<?php

use Doctrine\Common\Collections\Collection;
use Doctrine\Common\Persistence\ObjectManager;

class ObjectSerializer
{
    private $om;

    public function __construct(ObjectManager $om)
    {
        $this->om = $om;
    }

    public function serialize($object, $maxDepth = 1)
    {
        $visited = array();

        return $this->doSerialize($object, $visited, $maxDepth);
    }

    private function doSerialize($object, array &$visited, $maxDepth)
    {
        if ($object === null) {
            return;
        }

        $oid = spl_object_hash($object);
    
        if (!isset($visited[$oid])) {
            $visited[$oid] = 0;
        }

        $visited[$oid]++;

        if ($visited[$oid] > $maxDepth) {
            return;
        }

        $metadata = $this->om->getClassMetadata(get_class($object));

        $serialized = array();

        foreach ($metadata->getFieldNames() as $fieldName) {
            $serialized[$fieldName] = $metadata->getFieldValue($object, $fieldName);
        }

        foreach ($metadata->getAssociationNames() as $associationName) {
            $associationValue = $metadata->getFieldValue($object, $associationName);

            if ($associationValue === null) {
                continue;
            }

            if ($metadata->isCollectionValuedAssociation($associationName)) {
                if ($associationValue instanceof Collection || is_array($associationValue)) {
                    foreach ($associationValue as $collectionObject) {
                        $serialized[$associationName][] = $this->doSerialize($collectionObject, $visited, $maxDepth);
                    }
                }
            } else {
                $serialized[$associationName] = $this->doSerialize($associationValue, $visited, $maxDepth);
            }
        }

        return $serialized;
    }
}