“对象引用未设置为对象的实例”错误与php soapclient

Question

我知道有一些这样的帖子，但我还没有看到解决我问题的帖子。我打电话给kashflow api插入发票。它需要一个“Invoice”类型的对象，而该对象又需要一组InvoiceLine类型的对象。

见这里：

http://www.kashflow.com/developers/soap-api/invoice/

http://www.kashflow.com/developers/soap-api/InvoiceLine/

以下是我目前正在使用的代码。

class Invoice{

    public $InvoiceDBID = ''; //TSystem-wide unique number for this invoice (used internally only)
    public $InvoiceNumber = ''; //Unique invoice number
    public $InvoiceDate = '';   //Invoice date
    public $DueDate = '';   //Invoice due date
    public $SuppressTotal = ''; //1 to suppress the total (only used on Quotes)
    public $ProjectID = ''; //The project ID for this invoice. 
    public $CurrencyCode = '';  //The currency code to be used for the invoice e.g. USD, GBP, ZAR
    public $ExchangeRate = '';  //The exchange rate for the currency used. 
    public $Paid = 0;   //1 for Paid, 0 for Unpaid. Set to 0 on Insert calls
    public $CustomerID = '';    //The ID of the customer (or supplier int he case of receipts)
    public $CustomerReference = ''; 
    public $EstimateCategory = '';  //The category of the quote (only relevant when with Quotes)
    public $NetAmount = 0;  //The total net amount of the invoice
    public $VatAmount = 0;  //The total VAT amount of the invoice
    public $AmountPaid = 0; //The sum of all payments made to this invoice
    public $Permalink = ''; //A permanent link to access a PDF of the invoice.
    public $Lines = ''; //A collection of type InvoiceLine.

}

class InvoiceLine{

    public $LineID = ''; //System-wide unique number for this line
    public $Quantity   = ''; //Quantity invoiced
    public $Description   = ''; //Description of item invoiced 
    public $Rate = '';  //Rate per item
    public $ChargeType = '';    //For Purchases: Related Nominal ID From GetNominalCodes.
    public $ProjID = '';    //The Project to which this line should be assigned. Leave as 0 if in doubt 
    public $VatAmount = ''; //The VAT amount for this line
    public $VatRate = '';   //The VAT rate applied to this line (e.g. 15 for 15%). For Vat Exempt (the n/a code) use -1
    public $Sort = '';  //Sort order value
    public $ProductID  = '';    //An integer value representing the ID of a SubProduct (from GetSubProducts). Set to 0 for “None”
    public $ValuesInCurrency = '';  //Set to “1? if the Rate and VATAmount is already in foreign currency

}

$client = new SoapClient("https://securedwebapp.com/api/service.asmx?WSDL");    

$invoice_line = new InvoiceLine();
$invoice_line->Quantity = 1;
$invoice_line->Description = 'Delicious Irish Whisky';
$invoice_line->Rate = 20;
$invoice_line->ChargeType = 5614031;
$invoice_line->VatAmount = 4.00;
$invoice_line->VatRate = 20;
$invoice_line->ProductID = 618736;

$invoice = new Invoice();
$invoice->InvoiceDate = '11/10/2012';
$invoice->DueDate = '08/11/2012';               
$invoice->CurrencyCode = 'GBP';
$invoice->ExchangeRate = 1.0000;
$invoice->CustomerID = '1780661';
$invoice->Lines = array($invoice_line);


$parameters['UserName'] = "xxxxxxxxx";
$parameters['Password'] = "xxxxxxxxx";
$parameters['Invoice'] = $invoice;
$response = $client->InsertInvoice($parameters);