DTD validation problem

We parse our XML using the PHP SAX functions (e.g., xml_parse(), etc.).  The
XML we receive might take the form:

     <USERDATA>
          <YOURNAME>Will Smith</YOURNAME>
          .
          .
     </USERDATA>

Before we pass this data to our parser, we prepend a string representing the
DTD we use to validate the data.  Based on my example data, the
corresponding DTD would take the form:

     <?xml version="1.0"?>
     <!DOCTYPE USERDATA [

     <!ELEMENT USERDATA (YOURNAME, ...)>
     <!ELEMENT YOURNAME (#PCDATA)>
     .
     .

     <!ATTLIST YOURNAME maxlength CDATA #FIXED "40">
     .
     .

     ]>

Note that we are defining an attribute called "maxlength" with a default
("FIXED") value of "40".  When the combined string containing the DTD and
the XML data is parsed, we check that the value of YOURNAME does not exceed
the predefined number of characters.  If it does, we return an error.

Under PHP 4, xml_parse() reads the data above and populates the attributes
array for YOURNAME just as it would if the user had passed:

     <YOURNAME maxlength="40">Will Smith</YOURNAME>

Under PHP 5, however, xml_parse() appears to ignore default attributes
altogether.  As a result, the attributes array for YOURNAME is empty.

This is a big problem for us because our method of validation is essentially
defeated under PHP 5.  I assume this difference in behavior between PHP 4
and 5 can be ascribed to the differences between the expat and libxml2
libraries in each version respectively.  That having been said, we are
considering migrating to PHP 5 for various reasons, but before we do, we
need to know if there is a way to force our SAX parser to recognize default
attributes under PHP 5.

Any help is very much appreciated!