PHP JSON Array Objects adding 0 as key

UPDATED QUESTION

Here is my class...

class EbayItem
{
    public $ebayItemId;
}
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Right before my foreach loop, I have the following:

$mainResponse = array();
$mainResponse = [
    [
        "ack" => "success",
    ],
    "result" => []
    ];
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Inside of my loop, I have...

$ebayItem = new EbayItem();
$ebayItem->ebayItemId = $itemId;
array_push($mainResponse['result'], $ebayItem);
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Right after my loop I have...

echo json_encode($mainResponse);
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This will produce the following:

{"0":{"ack":"success"},"result":[{"ebayItemId":{"0":"153532419741"}}]}
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I need it to produce...

{"ack": "success","result": [{"ebayItemId": "153532419741","ebayItemTitle": "..."}]}
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