Fatal error: SOAP-ERROR: Encoding: object hasn't 'Qualification' property

I'm having a bit of a problem and I was wondering if someone could help
out.

 

I'm creating a SOAP client:

 

$client = new SoapClient($url, array('trace' => 1,'exceptions' => 0));

$result = $client->OpGetList(array($qualification));

 

After I call OpGetList from the web service I receive this:

 

Fatal error: SOAP-ERROR: Encoding: object hasn't 'Qualification'
property

 

If I call the same service with a string instead of an array:

 

$result = $client->OpGetList("foo");

 

I get this:

 

<SOAP-ENV:Body>

<ns1:OpGetList/>

</SOAP-ENV:Body>

</SOAP-ENV:Envelope>

 

But it should look like this:

 

<SOAP-ENV:Body>

<ns1:OpGetList>

<ns1:Qualification>some string</ns1:Qualification>

</ns1:OpGetList>

</SOAP-ENV:Body>

</SOAP-ENV:Envelope>

 

 

I know the WSDL works, because it works in saopUI without any issues.

 

Why isn't the soapClient able to create the correct soap request?

 

Thanks in advance for your help.