Hi,
I have been a PHP developer for 13 years and have experience building enterprise applications.

What framework are you using? Most PHP frameworks have a built in authentication and authorization mechanism. For example, Yii has a builtin RBAC system that allow you to define a hierarchical permission system.

If you're not using a framework or you just want to implement your own system for whatever reason then you can utilize Object Oriented Design and implement a Base controller (if MVC) that checks the permission for each request. That won't use a lot of if statements.

Probably need more info, to give definitive guidance. Hope that helps.

Your business needs a strong technical team working on your project consistently. So its natural to be tempted to partner with a technical person who you may be able to offload the demand of ensuring the technical details are adequately implemented and maintained. However, they must bear risk. In your case, the person you're considering seems to be demanding stake plus payment for his services. Your vision is to grow your business, his maybe to get rich. If you're confident in their ability to develop and maintain then I'd suggest keeping it that way. The worst thing you'd want in your startup is having partner disputes early on. Also, no one developer can realistically handle all the tasks you identified. For the social media, online marketing and email marketing I'd hire a tech savvy marketer with a proven track record. For the technical bits, you can build a team around the core developer according to how far your budget can stretch. You may be able to find junior or inexperienced talent to work along with your senior experienced core developer. Whatever you do though, ensure that you can consistently do marketing and sales.