Middleware is the code layer for handling common requests and responses in PHP web applications. Its core functions include authentication, logging, CORS settings and input filtering. 1. The middleware is located between the request and the response, and is processed layer by layer by layer using the "Onion Model"; 2. It is often used to uniformly handle non-business logic tasks; 3. Logic is implemented through the handle() method in frameworks such as Laravel; 4. Unlike the controller, the middleware handles global operations, while the controller focuses on specific business logic; 5. When writing, you need to pay attention to calling the next middleware, avoiding complex operations, keeping the order correct, and not directly outputting the content.

PHP middleware is actually a very practical concept in web applications. It is not a new thing, but it has become more and more common with the development of modern frameworks. You can understand it as a series of "processing layers" between the request and the final response, with each middleware having the opportunity to modify the request or response content, or decide whether to continue passing.

What is middleware?

Middleware is essentially a piece of code that comes before the web request enters the final processing logic (such as the controller method), and before the response is returned to the browser. It is used to handle common tasks such as:

• Authentication
• Logging
• Request filtering
• Cross-domain settings

Its design idea is "onion model": the request passes through each layer of middleware from the outside to the inside, and then returns the response from the inside to the outside after processing.

What can middleware do?

Middleware is very suitable for doing things that do not require specific business logic to participate but also need to be handled uniformly in multiple requests. Here are several common usage scenarios:

• Authentication : Check whether there is a valid Token or Session, if there is no, it will directly return 401.
• Logging : Records the URL, IP, time-consuming and other information of each request.
• CORS settings : Add cross-domain related fields in the response header.
• Input filtering : Do some preprocessing of request parameters, such as cleaning up XSS input.

To give a simple example: if you want all API requests to bring a specific header, you can write a middleware to intercept requests that do not meet the criteria and directly return an error without repeating judgments in each interface.

How to use middleware in PHP framework?

Most modern PHP frameworks (such as Laravel, Slim, and Symfony) support middleware mechanisms. Although the implementation methods are slightly different, the basic structures are similar.

Taking Laravel as an example, creating middleware is simple:

 php artisan make:middleware CheckToken

Then you will get a class with two methods: handle() and terminate() . The main logic is written in handle() , such as:

 public function handle($request, Closure $next)
{
    if (! $request->headers->has('Authorization')) {
        return response('Unauthorized.', 401);
    }

    return $next($request);
}

Finally, register this middleware in Kernel.php, or specify use in the route.

Tips:

• Some middleware is only suitable for specific routes and can be bound to a routing group.
• If there is a lot of logic in the middleware, it is recommended to split it into multiple small middleware, which is clearer and easier to reuse.

The difference between middleware and controller

Many novices tend to confuse the functions of middleware and controllers. Simply put:

• The controller is responsible for handling specific business logic, such as reading the database and generating page content.
• Middleware is responsible for handling global and universal operations and does not care about specific business.

So, if you find that a certain logic recurs in multiple controllers, it may be more suitable to put it in middleware.

What should you pay attention to when writing middleware?

There are a few points to pay special attention to when writing middleware:

• Make sure to call $next($request) or the request will be stuck.
• Don't do too complex calculations in middleware, as they will affect performance.
• The order is very important! The order of execution of middleware is the order in which they are defined.
• Avoid directly outputting content in middleware, it is best to return by modifying the response object.

Basically that's it. Middleware is not a complex concept, but is very useful in actual development, especially when building maintenance-based, modular applications. As long as you master the timing and boundaries of its use, your code will be cleaner and more organized.

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