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.
h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=c03360041
securityreason.com/securityalert/8525
svn.php.net/viewvc/?view=revision&revision=317183
www.byte.nl/blog/2011/09/23/security-bug-in-is_a-function-in-php-5-3-7-5-3-8/
www.securityfocus.com/archive/1/519770/30/0/threaded
bugs.php.net/bug.php?id=55475
bugzilla.redhat.com/show_bug.cgi?id=741020