The is_a function in PHP 5.3.7 and 5.3.8 triggers a call to the __autoload function, which makes it easier for remote attackers to execute arbitrary code by providing a crafted URL and leveraging potentially unsafe behavior in certain PEAR packages and custom autoloaders.
https://bugzilla.redhat.com/show_bug.cgi?id=741020
http://www.securityfocus.com/archive/1/519770/30/0/threaded
http://svn.php.net/viewvc/?view=revision&revision=317183
http://securityreason.com/securityalert/8525
http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=c03360041