This is a simple little script written by sam marshal and others at the Open University to help ensure that your code follows some of the basic rules from the Moodle coding style guidelines.
At the moment, the checks are very basic (they mostly just use simple regular expressions) so treat the output from this script as suggestions for bits of code you may wish to review, not as a definitive list of problems you must fix.
Download the zip (or clone using git). Rename the folder you get to <tt>codechecker</tt>, and copy it into the <tt>local</tt> folder of your Moodle install.
You should then find a new entry Site administration -> Development -> Code checker in the settings block.
https://github.com/timhunt/moodle-local_codechecker/tree/experiment - the version uses the more sophisticated PHP CodeSniffer library to do better checks.