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