soap calling parametric function

See more:

PHP

i am call a web service that have a function that have parameter but on calling give the error
this is my call request
$ccode = $client->GetISD('a');

this is a function

<s:element name="GetISD">
<s:complexType>
<s:sequence>
<s:element minOccurs="0" maxOccurs="1" name="CountryName" type="s:string"/>
</s:sequence>
</s:complexType>
</s:element>