The PHP MVC (Model-View-Controller) design pattern is a architectural pattern that separates the application logic into three interconnected components – Model, View, and Controller.

1. Model: The model represents the data and business logic of the application. It handles the data storage, retrieval, and manipulation. In PHP, the model is typically implemented as classes that interact with a database or other data sources.

2. View: The view is responsible for displaying the data to the user. It presents the data retrieved from the model in a format suitable for the user interface. In PHP, the view is usually implemented as HTML templates that include dynamic data using placeholders or templating engines.

3. Controller: The controller is the intermediary between the model and the view. It receives user input and decides how to handle it. It interacts with the model to retrieve and manipulate the data, and it passes the processed data to the view for display. In PHP, the controller is implemented as classes that handle user requests and decide which view to render.

The flow of the application in PHP MVC design pattern is as follows:
1. The user interacts with the user interface (typically through a web browser) and triggers a request.
2. The request is received by the controller, which decides how to handle it based on the user input.
3. The controller interacts with the model to retrieve or manipulate the data.
4. The processed data is passed to the view, which renders the HTML output.
5. The HTML output is sent back to the user interface for display.

Advantages of using the PHP MVC design pattern:
1. Separation of concerns: The MVC design pattern separates the application logic into three distinct components, making the code easier to understand and maintain.
2. Code reusability: The modular structure of MVC allows for code reusability and promotes the use of best practices.
3. Scalability: The MVC design pattern allows for easy scaling of the application by adding or modifying components as necessary.
4. Testability: The MVC design pattern facilitates unit testing of individual components, making it easier to identify and fix bugs.

Overall, the PHP MVC design pattern helps in structuring PHP applications in a way that is flexible, maintainable, and scalable. It promotes code reusability, separation of concerns, and testability, making it a popular choice for developing web applications.