PHP json_decode() is turning an integer value into bool(true)

I am processing a json string using $input = json_decode($raw, true); and feeding values into an array with the objective of outputting that array as a purchase order receipt.

It reads most of the json fine, except the "product_id" within the first (and in this case only) line item array of "line_items" (sample below). "line items" is itself a sub-array of "order".

Whilst it reads most of the array $input["order"]["line_items"][0] OK, it interprets

json string sample:

        "line_items": [{
        "id": 124,
        "subtotal": "332.50",
        "subtotal_tax": "66.50",
        "total": "0.00",
        "total_tax": "0.00",
        "price": "0.00",
        "quantity": 1,
        "tax_class": null,
        "name": "Hydraflex Mattress",
        "product_id": 1037,
        "sku": "",
        "meta": [{
            "key": "pa_size",
            "label": "Available Sizes",
            "value": "UK Standard Single"
        }]

Result of var_dump($input["order"]["line_items"][0]);:

array(12) { 
    ["id"]=> int(124) 
    ["subtotal"]=> string(6) "332.50" 
    ["subtotal_tax"]=> string(5) "66.50" 
    ["total"]=> string(4) "0.00" 
    ["total_tax"]=> string(4) "0.00" 
    ["price"]=> string(4) "0.00" 
    ["quantity"]=> int(1) 
    ["tax_class"]=> NULL 
    ["name"]=> string(18) "Hydraflex Mattress" 
    ["product_id"]=> bool(true) 
    ["sku"]=> string(0) "" 
    ["meta"]=> array(1) { 
        [0]=> array(3) { 
            ["key"]=> string(7) "pa_size" 
            ["label"]=> string(15) "Available Sizes" 
            ["value"]=> string(18) "UK Standard Single" 
        } 
    } 
} 

Main code:

$prices = array(
    5928 => 34.48, //Sleepsac;
    5924 => 99.99,
    5925 => 99.99,
    5926 => 99.99,
    5927 => 99.99,
    1037 => 105.67
);

$raw = file_get_contents("webhook_sample.json");
$input = json_decode($raw, true);

// create an initial array with required fields set to null
$output = array(
    "po_number" => NULL, 
    "item" => NULL,
    "quantity" => NULL,
    "invoice_price" => NULL,
    "invoice_address" => NULL,
    "delivery_address" => NULL,
    "instructions" => NULL
);

function mailout($message) {
    $to      = '[email protected]';
    $subject = 'the subject';
    $headers = 'From: [email protected]' . "\r\n" .
        'Reply-To: [email protected]' . "\r\n" .
        'X-Mailer: PHP/' . phpversion();

    mail($to, $subject, $message, $headers);
}

// provides a check to see if the field is set and whether it's blank.
function conditional_return($input, $multi_input = false, $multi_last = false) {
    if (isset($input)) {
        if (!empty($input)) {
            if ($multi_input && !$multi_last) {
                // if it's not the last one
                return $input.', ';
            } else {
                // if it is the last one or if it's a solo entry
                return $input;
            }
        } else {
            return "";
        }
    } else {
        return "";
    }
}

// create and populate array for address
$shipping_address = array();
$shipping_address["address_1"] = $input["order"]["shipping_address"]["address_1"];
$shipping_address["address_2"] = $input["order"]["shipping_address"]["address_2"];
$shipping_address["city"] = $input["order"]["shipping_address"]["city"];
$shipping_address["state"] = $input["order"]["shipping_address"]["state"];
$shipping_address["postcode"] = $input["order"]["shipping_address"]["postcode"];

// strip out any address elements that are not populated
foreach ($shipping_address as $key => $value) {
    if ($value == "") {
        unset($shipping_address[$key]);
    }
}
// use remaining elements to create text address
$shipping_address_string =  implode(", ", $shipping_address);

$number_of_items = count($input["order"]["line_items"]);

for ($i=0; $i < $number_of_items; $i++) { 
    if ($input["order"]["line_items"][$i]["product_id"] = 1037 || 
        $input["order"]["line_items"][$i]["product_id"] = 0000 ||
        $input["order"]["line_items"][$i]["product_id"] = 0000 ||) {

            // Set the output parameters
            echo "variable i is ".$i."<br>";
            setlocale(LC_MONETARY,"en_EN");
            $output["item"] = $input["order"]["line_items"][$i]["name"];
            $output["quantity"] = $input["order"]["line_items"][$i]["quantity"];
            $output["invoice_price"] = $prices[$input["order"]["line_items"][$i]["product_id"]];
            $output["invoice_address"] = "Mammoth Technologies, Office 3, Yarm Road, Stockton-on-Tees, TS18 3NA, UK";
            $output["delivery_address"] = $shipping_address_string;
            $output["instructions"] = $input["order"]["note"];

            // write output string
            $output_string = "";
            foreach ($output as $key => $value) {
                $output_string .= $key.": ".$value."<br>";
            }
            echo $output_string;
            var_dump($prices);
            echo "<br><br>";
            echo($input["order"]["line_items"][0]["product_id"]); 
            echo "<br><br>";
            var_dump($input["order"]["line_items"][0]);
            // mailout($output_string);
            // file_put_contents("/outbox/dermatherapy_PO_".$input["order"]["id"]."_".$input["order"]["line_items"][$i]["product_id"].".txt", $output_string);
    }