Volker Dusch has a suggestion for all of the PHP projects (or, really Open Source projects in general) that can help keep things cleaner in your codebase and make for simpler times when merging contributions - including your coding standard along with the rest of your project.

Let me elaborate on [an important] point: Contribution. Most developers i know care about producing good code, especially then they are contributing to an open source project! Those people will respect your coding standard, naming scheme and every thing else that they can check for before sending you all patch/pull request. So try to make that part easy.

He talks about doing things the hard way - reformatting everything by hand each time someone contributes - or the easier way of enforcing the coding standard as a part of the contribution flow. He mentions PHP_CodeSniffer and the PHP Mess Detector as a part of a Jenkins installation (easily built from this handy project).