In case someone finds this, and having the same issue.
This user should have done the code differently, he specified the creation of a CData node, and then tried creating another inside the existing. This example is trivial, but hopefully shows what went wrong.
His example implied he wanted data wrapped in CData, and something extra left over. Who truly knows, but I'm giving the correct syntax anyway.
End result of running that would be
// create the xmlwriter object, etc.
// First example, create the node along with the data.
$xw->writeCData('data inside the CData');
// Second example, create the node and also add data, not forgetting to close it.
$xw->endElement(); // End description
// close the xmlwriter object, and output
<description><![CDATA[data inside the CData]]>fill text</description>