Try to add an invoice from php using curl but I get " XMLString should be a part of the request" error.

My code is simple

2. $query = urlencode('authtoken='.AUTHTOKEN.'&apikey='.APIKey.'&SCOPE'.scope.'&XMLString='.$xml);
3. $x = curl_init("https://invoice.zoho.com/api/invoices/create");
4. curl_setopt($x, CURLOPT_HEADER, 0);
5. curl_setopt($x, CURLOPT_POST, 1);
6. curl_setopt($x, CURLOPT_POSTFIELDS, $query);
7. curl_setopt($x, CURLOPT_FOLLOWLOCATION, 0);
8. curl_setopt($x, CURLOPT_RETURNTRANSFER, 1);
9. $data = curl_exec($x);
10. curl_close($x);
11. echo $data;

The xml sent is a valid one:

2. <Invoice>
3.       <CustomerID>20</CustomerID>   
4.       <InvoiceDate>2011-09-27</InvoiceDate>
5.       <InvoiceNumber>TUBI20</InvoiceNumber>   
6.       <PONumber>20</PONumber>
7.       <ExchangeRate>1</ExchangeRate>  
8.       <LFName>Flat fee daily</LFName>
9.       <PaymentGateways>
10.             <PayPal>0</PayPal>   
11.       <Authorize.Net>false</Authorize.Net>    
12.       <GoogleCheckout>false</GoogleCheckout>    
13.       </PaymentGateways>
14.       <InvoiceItems>
15.             <InvoiceItem>
16. <ItemName>01019112000BR </ItemName>   
17. <ItemDescription>F.40 NON KAT LOUD EXHAUST + BORDINO </ItemDescription>
18. <Price>4051.14</Price>
19. <Quantity>1</Quantity>
20.       </InvoiceItem>     
21.       </InvoiceItems>
22.       <Comments>
23.             <Comment><Description></Description></Comment>
24.       </Comments>
25.       <Notes></Notes>
26.       <Terms></Terms>
27. </Invoice>

Can anyone help me please?

Thank you!