Passing a PHP Object as argument of a javascript function

My searching skills seems to have failed me. I have this php object that I unserialize from a mysql entry, and I want to pass it as an argument to a javascript function, so it could compare the object with the values in a form. From what I gathered from my search, encoding the object as a json object would have done the trick, but when I'm doing a json_encode on the variable, it only result in {}.

Here is the relevant snippet of code:

<?php
$data = new Data();
$data = unserialize(base64_decode($rawdata));//Where $rawdata is the data retrieved from the mysql query.
/* using function such as $data->getName() to retrieve the relevant data */
?>

<form id="myform" action="#" method="post" onsubmit="compareEntry(<?=json_encode($data)?>)">

<!-- Different input and select field initialized with the php data -->

<input type="submit" onclick="compareEntry(<?=json_encode($data)?>)"/>
</form>
<!--<?=json_encode($data)?>--> 

I know that the php data is correctly retrieved from the database, as the values in the form are all correctly initialized. Only with the last html comment did I knew that I had an empty json object.

Here is an example of what print_r($data) returns (sensitive information edited):

(
    [m_path:private] => 
    [m_version:private] => REL_54
    [m_bugs:private] => Array
 *RECURSION*
    [m_targets:private] => Array
 *RECURSION*
    [m_symptoms:private] => Array
 *RECURSION*
    [m_exception:private] => Array
 *RECURSION*
    [m_instruction:private] => Array
 *RECURSION*
    [m_sources:private] => Array
 *RECURSION*
    [m_risks:private] => Array
 *RECURSION*
    [m_test:private] => Array
 *RECURSION*
    [m_contact:private] => Array
 *RECURSION*
)
1

Do I do something wrong? Is encoding to JSON the right approach in my scenario?