If you want to use the WDDX 1.0 DTD for validation (http://www.openwddx.org/downloads/dtd/wddx_dtd_10.txt) check that you define the comment as element, not as attribute.
<?xml version="1.0" encoding="iso-8859-1" ?>
<wddxPacket version='1.0'>
<header>
<comment>PHP</comment>
</header>
<data><!-- WDDX serialized data --></data>
</wddxPacket>
