The PHP Framework Interop Group (FIG) has defined a standard for middleware interfaces in PHP. That's where PSR-15 comes in.

PSR-15 is a PHP Standard Recommendation that provides a set of interfaces for HTTP middleware components. These interfaces make it easier to create reusable middleware that can be used across different PHP frameworks and applications. In this video, we'll explore the PSR-15 standard in detail, including the key interfaces, how to create and use middleware, and how it can help you build more modular and flexible PHP applications.