Scumworks SOAP API with PHP

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Somebody PLEASE help! I've spent all day trying to createBacklogItem() from PHP. I keep getting a java.lang.NullPointerException.

Let's say I have a product, "Product A", with productId = 1.
Let's say I have a release, "Release A", with releaseId = 2.

How do I now add a backlog item to "Release A"?

I have tried the following (tried to fill all input params):

$createBacklogItem = new createBacklogItem();
$createBacklogItem->backlogItem = new backlogItem();
$createBacklogItem->backlogItem->businessWeight = new businessWeight();
$createBacklogItem->backlogItem->id = 123123123;
$createBacklogItem->backlogItem->key = "SOMEKEY";
$createBacklogItem->backlogItem->name = "TestingTestingTesting";
$createBacklogItem->backlogItem->active = true;
$createBacklogItem->backlogItem->description = "Test from SOAP API call.";
$createBacklogItem->backlogItem->estimate = 20;
$createBacklogItem->backlogItem->businessWeight->benefit = 0;
$createBacklogItem->backlogItem->businessWeight->penalty = 0;
$createBacklogItem->backlogItem->completedDate = date("c");
$createBacklogItem->backlogItem->productId = $productId;
$createBacklogItem->backlogItem->releaseId = $releaseId;
$createBacklogItem->backlogItem->sprintId = null;
$createBacklogItem->backlogItem->rank = 0.0;
$createBacklogItem->backlogItem->theme = new theme();

$backlogItem = $service->createBacklogItem($createBacklogItem);

No matter what I do, I just keep getting a NullPointerException

$service is fine, as I am able to call $service->getProductByName($productName) and $service->getReleasesForProduct($getReleasesForProduct) etc.

I have looked at the:

public void testCreateBacklogItem()

method in the example Java code (in /opt/scrumworks-5.0.0/apiclient on my Ubuntu system) seems to use type BacklogItemWSO, but this seems to be burried in a .class file in the .jar (I've never touched Java before).

Any help would be really appreciated.