gistfile1.php

<?php

public function getEntryJson(EntryModel $entry)
{
    $entryData = array();

    foreach ($entry->getType()->getFieldLayout()->getFields() as $field)
    {
        $field = $field->getField();
        $handle = $field->handle;
        $value = $entry->$handle;

        if ($field instanceof BaseElementFieldType)
        {
            $entryData[$handle] = array();
            
            foreach ($value as $relElement)
            {
                $entryData[$handle][] = array(
                    'id' => $relElement->id,
                    'label' => (string) $relElement
                );
            }
        }
        else if ($field instanceof MatrixFieldType)
        {
            $entryData[$handle] = array();

            foreach ($value as $block)
            {
                $entryData[$handle][] = array(
                    // ...
                );
            }
        }
        else
        {
            // Deal with Checkboxes and Multi-select fields
            if ($value instanceof \ArrayObject)
            {
                $value = array_merge($value);
            }

            if (is_array($value))
            {
                $entryData[$handle] = $value;
            }
            else
            {
                // Let's just force anything else to a string, in case it's something like a SingleOptionFieldData class
                $entryData[$handle] = (string) $value;
            }

            $entryData[$handle] = $value;
        }
    }

    return JsonHelper::encode($entryData);
}