Webservice fast license manager not accepting request

I want to use Quick License Manager SOAP webservice to connect e-commerce wordpress plugin to allow user to buy license and show it after successful purchase.
So now I am trying to call the GetProductInfo function to get information about an existing product that I created using the QLM Management Console.
I am using the PHP SoapClient class, but when I try to make a request, I get this response:

Validation request: The input is not a valid Base-64 string because it contains a non-base 64 character, more than two indents, or an invalid character among the padding characters., Caller: GetProductInfo

I don't understand what happened to my request, so I need help ... Here is the code I'm using:

<?php 
$url = "https://quicklicensemanager.com/****/qlm/qlmservice.asmx";
$wsdl = $url . "?WSDL";
$soapClient = new SoapClient($wsdl, array(
    "encoding" => "utf8",
    "trace"    => TRUE,
    "version"  => SOAP_1_1
));
$req = sprintf(
'<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Header>
    <QlmSoapHeader xmlns="http://www.interactive-studios.net/qlmweb">
      <CultureName>%s</CultureName>
      <UtcOffset>%d</UtcOffset>
    </QlmSoapHeader>
  </soap:Header>
  <soap:Body>
    <GetProductInfo xmlns="http://www.interactive-studios.net/qlmweb">
      <eProductName>%s</eProductName>
      <productID>%d</productID>
      <major>%d</major>
      <minor>%d</minor>
    </GetProductInfo>
  </soap:Body>
</soap:Envelope>', "en_US", 0, "The IO plug-in: Student License", 4, 1, 0);
echo '<pre>';
print_r(htmlentities($req));
echo '</pre>';

$action = "http://www.interactive-studios.net/qlmweb/GetProductInfo";

$res = $soapClient->__doRequest($req, $url, $action, 1);

echo '<pre>';
print_r($res);
echo '</pre>';
echo "=======================================================";
echo '<pre>';
print_r(htmlentities($soapClient->__getLastRequest()));
echo '</pre>';
echo "=======================================================";
echo '<pre>';
print_r($soapClient->__getLastRequestHeaders());
echo '</pre>';

      

        
        
        
      

    

Thanks in advance for your help.

EDIT Now I tried to change the product field id and it seems that this is the one that was wrong. So it should be a base64 string, so I updated my code:
  </soap:Envelope>', "en_US", 0, "The IO plug-in: Student License", 4, 1, 0);

becomes</soap:Envelope>', "en_US", 0, base64_encode("The IO plug-in: Student License"), 4, 1, 0);

New error message:

Validation request: The length of the data to decrypt is invalid. Caller: GetProductInfo