Signing JSON objects

I have to exchange JSON objects between different platforms and implementations of a service and make its integrity verifiable via digital signatures. So a platform A would create such an object and create a digital signature. Said signature is then included into the object and sent to platform B. The JSON objects can contain arbitrary attributes and data.

E.g. in PHP:

function signObject($jsonObjectToSign, $privateKey) {
    $jsonObjectToSign->signature = "";
    $msgToSign = json_encode($jsonObjectToSign);

    openssl_sign($msgToSign, $jsonObjectToSign->signature, $privateKey, OPENSSL_SLGO_SHA1);

    return $jsonObjectToSign;
}

Problem is, that e.g. in Java, there is no way to tell whether the attributes of a JSON object will be in the same order you added them (via JSONObject.put()). So, if I do a

$json = json_encode('{"a":1, "b":2}');

in PHP, sign this object as stated above, transfer it to a java based server, decode the json object and then try to verify the signature, I'd probably get a different order of the object's attributes.

So what I need, is a reliable way to create a String from a JSONObject, independent of the language or platform used.

The example object above needs always to output {"a":1, "b":2} and NEVER {"b":2, "a":1}. Unfortunately, this is the usual case e.g. in Java.

Is there any "best practice" to sign JSON Objects in a secure way?

But let me describe the problem in another way:

Let's say I want to do this in Java (or any other language):

JSONObject j = new JSONObject();
j.put("a", 1);
j.put("b", 2);

Now, I need a serialization function, that outputs always the same string representation for this object, no matter how and with what language this object is created.