You need to learn the following (I would suggest in this order):
- Basic Object-Oriented Principles (such as inheritance, polymorphism, and encapsulation)
- The PHP language itself. Specifically, PHP 5.
- Database Design Principles such as tables, keys, relationships, normalization, etc.
- SQL - Structured (or Standard never can remember which) Query Language. Specifically learn the basics of select, insert, update, and delete queries.
- Good design principles and coding practices (you can find posts here on StackOverflow for one) such as dividing presentation and business logic.
- A Framework, Any Framework - this will help you become introduced to more advanced concepts of object-oriented design patterns and allow you to follow tutorials that will encourage good design and coding practices.
- Object-Oriented Design Patterns like MVC, Database Abstraction Models and the like
- Advanced SQL and other database stuff like triggers, stored procedures, and other functions.