How can the additional namespace be added to the SOAP request in PHP?

To add an additional namespace to a SOAP request in PHP, you can use the `SoapVar` class to create a new XML element with the desired namespace. Then, you can append this element to the SOAP request using the `SoapHeader` class.

// Create a new SoapClient instance
$client = new SoapClient("http://example.com/soap.wsdl");

// Create a new XML element with the additional namespace
$additionalNamespace = new SoapVar('<ns1:additionalElement xmlns:ns1="http://example.com"></ns1:additionalElement>', XSD_ANYXML);

// Add the element to the SOAP header
$header = new SoapHeader("http://example.com", "additionalElement", $additionalNamespace);
$client->__setSoapHeaders($header);

// Make the SOAP request
$response = $client->__soapCall("yourSoapFunction", $yourParams);