Curl XML Request Issue

I have a problem with sending a request to a remote api - the error message:

Object reference not set to an instance of an object

<?php 
    $xml_data ="<?xml version='1.0' encoding='utf-16' ?>
    <ChameleonIAPI>
    <Method>SearchVacancies</Method>
    <APIKey>apikey</APIKey>
    <UserName>user</UserName>
    <Filter>
    <Param Name='Permanent' Value='True' />
    <Param Name='Contract' Value='True' />
    <Param Name='Temporary' Value='True' />
    <Param Name='PageNo' Value='1' />
    <Param Name='PageSize' Value='10000' />
    <Param Name='TagIDCSV' Value='!AND215476' />
    </Filter>
    </ChameleonIAPI>";

    $action = "postxml";
    $authkey = "key";
    $authpassword = "pass";
    $URL = "http://path/to/url/PostXml.aspx";

    $ch = curl_init($URL);
    //curl_setopt($ch, CURLOPT_MUTE, 1);
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/xml'));
    curl_setopt($ch, CURLOPT_POSTFIELDS, "Action=".$action."&AuthKey=".$authkey."&AuthPassword=".$authpassword."&xml=".$xml_data);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 0);

    $output = curl_exec($ch);
    curl_close($ch);
?>