Security audits of PHP applications are usually performed on a source code basis. However sometimes vendors protect their source code by encrypting their applications with runtime (bytecode-)encryptors. When these tools are used source code analysis is no longer possible and because these tools change how PHP works internally, several greybox security scanning/fuzzing techniques relying on hooks fail, too.
about this event: http://events.ccc.de/congress/2008/Fahrplan/events/2678.en.html
