Be careful with encodings, the xmlrpc-decode function is rather strict. For example, the following response parse returns NULL :
<?xml version="1.0"?>
<methodResponse>
<params>
<param>
<value><string>a & b</string></value>
</param>
</params>
</methodResponse>
You should use entities :
<?xml version="1.0"?>
<methodResponse>
<params>
<param>
<value><string>a & b</string></value>
</param>
</params>
</methodResponse>
If your server does not encode responses properly, you may have to process responses before parse.