PHP 5.3.7 changed the behavior of the is_a() function, used to check if an object is an instance of a class, to call the __autoload() function. This causes a remote code execute problem when coupled with a standard library like PEAR that internally uses is_a to check if a returned variable is an Error object or not.
An unprotected __autoload() function that blindly includes based upon the variable it receives can be tricked into including a remote file by, for example, uploading a specially crafted file containing a link to a remote website.
Full explanation + code example has been posted on our website at http://www.byte.nl/blog/2011/09/23/security-bug-in-is_a-function-in-php-5-3-7-5-3-8/ and has been e-mailed to email@example.com
-- Cipriano Groenendal