Zend SOAP Server: domdocument.savexml 无效字符值

我用zend设置了一个简单的肥皂服务器。我用我找到的一个例子。我尝试了许多解决方案，但仍然收到错误：

This page contains the following errors:
error on line 2 at column 1: Extra content at the end of the document
Below is a rendering of the page up to the first error.

PHP日志说

Warning:  DOMDocument::saveXML() domdocument.savexml invalid character value in 
/usr/share/ZendFramework-1.11.11/library/Zend/Soap/Wsdl.php on line 506

这是我的代码：

ini_set("soap.wsdl_cache_enabled", 0); // disabling WSDL cache
require 'Zend/Loader/Autoloader.php';
require 'Zend/Soap/Server.php';
require 'Zend/Soap/AutoDiscover.php';
require 'Zend/Soap/Client.php';
$wsdl_uri = 'http://MY-URL/soap.php?wsdl=1';
$wsdl = new Zend_Soap_Autodiscover();
$wsdl->setClass('MeinWebservice');
if (isset($_GET['wsdl'])) {
    $wsdl->handle();
} else {
    $server = new Zend_Soap_Server($wsdl_uri);
    $server->setClass('MeinWebservice');
    $server->handle();
}
class MeinWebservice {
    /**
     * Gibt den Wert mal 10 zurück
     *
     * @param int $inputParam
     * @return int
     */
    public function test1($inputParam) {
        return $inputParam * 10;
    }
    /**
     * Addiert die Werte
     *
     * @param int $inputParam1
     * @param int $inputParam2
     * @return int
     */
    public function test2($inputParam1, $inputParam2) {
        return $inputParam1 + $inputParam2;
    }
}

我做错了什么？