It is no longer possible to add custom rules for PHP. We'll eventually add the support of writing custom rules in Java (as it is currently the case for Java and COBOL). See http://docs.codehaus.org/display/SONAR/Extending+Coding+Rules.
Note that if you think that your custom rules might benefit others, you can submit them on the developer list (http://www.sonarqube.org/resources/join-our-community/). If we find them valuable, we'll implement them.