It's fine if your purpose is to learn how to program. But if your primary goal is to build a successful website through which to launch a business, then this is not a good idea for the reason JMC outlines and others.
Because you describe yourself as a novice (and the terminology you use sort of reflects that, e.g. "the php script" instead of "the application", "the CMS", "the platform", etc.), you probably are unaware of many of the common pitfalls and best-practices in web development:
- Are you familiar with XSS, CSRF, SQL injection, and the vulnerabilities of the platform you're developing for?
- Are you familiar with MVC or similar patterns for organizing your code? Are you familiar with programming concepts such as OOP, separation of concerns, loose coupling, DRY, database normalization, etc.?
- Are you familiar with the common solutions to many web development concerns, such as a routing engine, a templating system, ORMs or a database access layer, plugin systems, caching engine, slugs, REST architecture, ACL, etc.
- Are you familiar with SEO, UX design, and web accessibility? How good is your HTML and CSS? Do you understand the need to separate content and presentation? Are you familiar with semantic markup? Do you know SEO/UX/accessibility best practices?
If you can't answer yes to all of these questions, then you're likely not up to the task of developing a web application that's going to be used in a production environment. And if you aren't familiar with good software development practices (e.g. commenting, top-down development, OOP, etc.) then you're probably gonna run into problems when your website grows beyond a simple "script".
And by then it'll be very costly to fix the problem. In the worst cases (which comes up more often than you might expect), you'll have to scrap the whole thing and start from scratch. And it'll be a nightmare for whoever has to port the content over to the new system.
It's much better to just get it done right the first time. Either use an off-the-shelf CMS and save yourself the time and money, or at least use an MVC framework that will force you to follow many best practices and also give you the benefit of the collective knowledge of hundreds of experienced professional developers.