Associative array as request-data in XML-RPC request

[eluser]hlz[/eluser]
It has been adressed before but i can't find a solid answer or a good solution. I want to post code below to a rpc-server:

Code:

$request = array(
    
    array(
                
        'userid'    => array('99','string'),
        'url'       => array('http://yoursite.com','string'),
        'email'     => array('[email protected]','string'),
        'lastname'  => array('Smith','string'),
        'firstname' => array('John','string')
    ),
    
    'struct'
);

This doesn't work as a request. It does however work when the same array is set as a server-response. Could anyone tell me why a struct or array type cannot be used as request data?

Edit: my question actually is: how does one pass key-value pairs to a xml-rpc-server if code above cannot be used?