One issue that we encountered when using Silverlight to access a PHP script deployed on an Apache Server relates to the use of clientaccesspolicy.xml and crossdomain.xml files. Officially, Silverlight first looks for a clientaccesspolicy.xml file and, if this isn't present, a crossdomain.xml file. However, when both files were present in the correct directory location we persistently got a 'System.Security.SecurityException' error. When we removed clientaccesspolicy.xml the Silverlight application connected with the PHP script. Thus, when connecting from Silverlight to PHP under an Apache Server, the rule of thumb would seem to be to use crossdomain.xml only.

For more details on clientaccesspolicy.xml and crossdomain.xml see http://tinyurl.com/njy2ox and http://tinyurl.com/m693n6.