The problem is that PHP has a string type which is also used to hold binary data and dates. But XML-RPC defines three separate types for strings, binary data, and dates. How do you tell how you want strings encoded? That's where this function comes in.
Suppose the XML-RPC method "log.data" took a string, a date, and a binary object. To tell XML-RPC that the date (which is a PHP string) is a really a date and that the binary data (which is also a PHP string) is really binary data, try:
$string = "My logging event.";
$date = "20030115T12:22:37"; // Must be this format
$binary = fread($fp, 128);
$xmlrpcReq = xmlrpc_encode_request("log.data", array($string, $date, $binary));
Note the reference passing in the calls to xmlrpc_set_type; that enables the function to change the values from strings into what xmlrpc_encode/_request expects (which are objects that include a bonus field that tells the desired XML-RPC type).