在php中为soap调用添加一个参数

I'm fairly new to soap calls but I have got a simple one working ok, using arrays.

$xml_array['searchName'] = "websvc";
$xml_Parray['name'] = "Cust Code";
$xml_Parray['value'] = "338877";
//   $Parray['searchParameter xsi:type="met:StringParameter"']=$xml_Parray;     //errors 
$Parray['searchParameter']=$xml_Parray;


$client = new WSSoapClient($wsdl, array('trace' => 1, 'exceptions' => 0));
$client->__setUsernameToken($UserName, $Password);
$client->__setLocation('http://127.0.0.1:80/demo/search');          

$xml_array['searchParameters'] = $Parray;

$response = $client->__soapCall("getDocuments", array($xml_array));

So this give me...

<searchName>websvc</searchName>
<searchParameters>
  <searchParameter>
    <name>Cust Code</name>
    <value>338877</value>
  </searchParameter>
</searchParameters>

BUT I now need to add additional text

xsi:type="met:StringParameter"

<searchName>websvc</searchName>
<searchParameters>
  <searchParameter xsi:type="met:StringParameter">
    <name>Cust Code</name>
    <value>338877</value>
  </searchParameter>
</searchParameters>

I simply tried

$Parray['searchParameter xsi:type="met:StringParameter"']=$xml_Parray;

but get a reponse

Fatal error: SOAP-ERROR: Encoding: object has no 'searchParameter' property

Can anyone help ?

Update. I have found out that what i am trying to do is add attributes!

So I have tried this

array("searchParameter" => array("_" => "$xml_Parray", "xsi:type"=>"met:StringParameter"));

But I'm not sure thats right either!