This xmlrpc_encode function doesn't generate the "methodResponse" tag required by some clients.
Using xmlrpc_encode_request seems better:
<?php
$xmlresponse = xmlrpc_encode_request(NULL, $phpresponse);
xmlrpc_encode(1) generate
<?xml version="1.0" encoding="utf-8"?>
<params>
<param>
<value>
<int>1</int>
</value>
</param>
</params>
xmlrpc_encode_request(NULL,1) generate
<?xml version="1.0" encoding="iso-8859-1"?>
<methodResponse>
<params>
<param>
<value>
<int>1</int>
</value>
</param>
</params>
</methodResponse>
?>