Developing complex, dynamic PHP business applications with a team of technical and business professionals is not a simple prospect. You'll be faced with many challenges, some of which won't be entirely technical in nature.
The business-planning framework that you choose to work with will certainly affect the direction in which your software project grows. Choosing a process that offers a strict yet comfortable structure will place limits on how dynamically your project can change, but conversely reward you with a paper trail and a detailed road map to follow. Taking a more dynamic approach to planning may, however, mitigate the risk associated with technical or business unknowns.
The choice of which approach to system planning best fits your project is not one to take lightly, and you should be familiar with all planning options before making your decision, regardless of your personal or professional preferences.