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.