You can easily use tools such as Behat and phpspec for practicing BDD in PHP. Regardless of the specific BDD tools, the question of how to do BDD “properly” arises. According to Dan North, initiator of the BDD philosophy, it should be be practiced as a “mutliple-stakeholder, agile methodology”. However, most open-source projects are not developed with an explicit agile methodology. Also, there are hardly any stakeholder roles that are clearly distinguishable from each other - often contributor and user are actually one and the same. So, in the case of open-source projects, you can question the benefit of BDD.