When developing PHP applications, what are the best practices for structuring files and directories to improve maintainability and scalability?

When developing PHP applications, it is important to follow a clear and organized file structure to improve maintainability and scalability. This can include separating different components of the application into separate directories, using namespaces to avoid naming conflicts, and following naming conventions for files and classes.

// Example directory structure for a PHP application
// /app
//   /Controllers
//   /Models
//   /Views
//   /Helpers
//   /Config
//   /public
//     index.php