Further elaborating on the question, I am a freelance developer, I have been adopting structured PHP for years, and now every time I work on a project I feel that I'm doing something wrong, or not doing the project in a professional way. I am currently in the process of learning MVC approach and testing my skills on CodeIgnator but I'm still not in a full grip of the approach. To make things short, is it still acceptable if I can still use structured PHP in the projects I work on? Or should I stop completely till I'm capable of building an application with MVC?
You can develop your web application without any MVC. MVC approach has a various advantage over using core PHP for eg.
1 Faster and well-managed web development
2 Online help as lots of people working on the same
3 Don't have worry much about complicated implementations (i.e. security, redirections)
4 Minimal changes required when upgrading to newer software
5 Industries look for the people who are proficient in frameworks as well as core programming language.