PHP versions prior to 5.3.1 suffer from from several bugs that may pose a security risk. Issues such as buffer overflows, arbitrary memory reads, and more have been addressed.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/83333/oCERT-2009-017.txt
Source: https://packetstormsecurity.com/files/83333/Open-Source-CERT-Security-Advisory-2009.17.html