The readfile function in PHP 4.4.4, 5.1.6, and 5.2.1 allows context-dependent attackers to bypass safe_mode restrictions and read arbitrary files by referring to local files with a certain URL syntax instead of a pathname syntax, as demonstrated by a filename preceded a “php://…/…/” sequence.
h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?lang=en&cc=us&objectID=c01056506
h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?lang=en&cc=us&objectID=c01086137
secunia.com/advisories/25423
secunia.com/advisories/25850
www.vupen.com/english/advisories/2007/1991
www.vupen.com/english/advisories/2007/2374
www.exploit-db.com/exploits/3573