On his code-diesel blog Sameer has posted a tutorial that introduces you to the PHP_CodeSniffer library and some simple examples of it in action.

A coding standard basically tells developers in what style they must write their code. If programmers work in a team than a coding standard ensures that each will be able to read the others code without any effort. [...] A tool which scans your source code and checks it against a standard would be quite a help. PHP_CodeSniffer is one such tool.

The PHP_CodeSniffer package allows you to check code (to "sniff" it) and ensure that it matches against a certain format. The tool doesn't do any testing outside of that, though - no unit testing or functionality checking.

Sameer includes a guide to getting the tool installed and using it to get the source files in a certain directory. He also points to a tutorial for developing your own coding standards.