I am having this exact same problem. I first create a user and then I try to add a credit card. It processes fine and even provides me with a valid CreditCard ID, but when I go to look at the user under Order->Credit Cards (and even create a Credit Card report) in Infusionsoft, nothing is there. Here is the code. It is a bit hardwired in right now, but still does not produce a credit card. (The output of this function produces an integer, like 159).

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function addCC_IS () {

global $client, $key;

$outp = 1; // set default return to 1 (ran ok)

#The contact variables sets up the data as it will be added to the database
$credit_c = array(
"ContactId" => "76825",
"NameOnCard" => "Joe Blow",
"CardNumber" => "5490123512361236",
"CardType" => "MasterCard",
"ExpirationMonth" => "02",
"ExpirationYear" => "2012"
);

#This sets up the call. DataService.add is the name of the service, and the second array
#is a list of the parameters
$call = new xmlrpcmsg("DataService.add", array(
php_xmlrpc_encode($key), #The encrypted API key
php_xmlrpc_encode("CreditCard"), #The table we are adding to
php_xmlrpc_encode($credit_c) #The data to be added
));

#This actually makes the call and stores the result in $result
$result = $client->send($call);
$blah = $result->value();
#Check to see if we have an error. If not, print out the results.
if($result->faultCode()) {
$outp = 'Code: '.$result->faultCode().' Summary: '.$result->faultString().'';
}

return $blah;

}