Is wordpress giving you -32700:parse error. not well formed? Or is xmlrpc.php bothering you while getting getCategories function XML RPC call?
I did and i’ve found the problem:

In the IXR_Message class, the class which I use and also is recommended to parse the XML response of xmlrpc.php, uses preg_replace in the function parse() (IXR_Message->parse();). In PHP5 there has been some restrictions introduced to recursive regular expressions with Perl. Some parameters has been introducted to your php.ini (backtrack limit) and one of them is preventing me to interpret the XML-feed I get from my WordPress. Perhaps this one is very big, but i really need it, so:

Change at the beginning of your script your settings for example:
ini_set('pcre.backtrack_limit', 10000000);

You will see that the IXR_Message class is happy with the result it gets from the first lines of its parse function.

If you want to test if the backtrack limit is bothering you use the preg_last_error() function:

if (preg_last_error() == PREG_BACKTRACK_LIMIT_ERROR) {
print 'Backtrack limit was exhausted!';

The code in the parse() function is:
$this->message = preg_replace('/<\?xml(.*)?\?'.'>/', '', $this->message);