pages using the php filter show no content

Try and create this page with HTML area (switch to 'code' mode of course):

<?php 
watchdog('ClickTest', 'Got a click', array());
?> 

Then set the page filter to PHP. Submit. When you come back to edit it next time the page body will be empty in either Code or WYSIWYG mode. The node version with the above contents will still be in the DB.