what is php middleware

#php middleware (middleware) is a closure and returns a closure.

Middleware provides a convenient mechanism for filtering HTTP requests entering the application, which can be divided into pre-middleware and post-middleware. Commonly used to verify whether the user is authenticated, add response headers (cross-domain), record request logs, etc. (Recommended learning: PHP programming from entry to proficiency)

##Example

<?php
// 框架核心应用层
$application = function($name) {
    echo "this is a {$name} application\n";
};
 
// 前置校验中间件
$auth = function($handler) {
    return function($name) use ($handler) {
        echo "{$name} need a auth middleware\n";
        return $handler($name);
    };
};
 
// 前置过滤中间件
$filter = function($handler) {
    return function($name) use ($handler) {
        echo "{$name} need a filter middleware\n";
        return $handler($name);
    };
};
 
// 后置日志中间件
$log = function($handler) {
    return function($name) use ($handler) {
        $return = $handler($name);
        echo "{$name} need a log middleware\n";
        return $return;
    };
};
 
// 中间件栈
$stack = [];
 
// 打包
function pack_middleware($handler, $stack)
{
    foreach (array_reverse($stack) as $key => $middleware) 
    {
        $handler = $middleware($handler);
    }
    return $handler;
}
 
// 注册中间件
// 这里用的都是全局中间件，实际应用时还可以为指定路由注册局部中间件
$stack[&#39;log&#39;] = $log;
$stack[&#39;filter&#39;] = $filter;
$stack[&#39;auth&#39;] = $auth;
 
$run = pack_middleware($application, $stack);
$run(&#39;Laravle&#39;);

Output ：

Laravle need a filter middleware
Laravle need a auth middleware
this is a Laravle application
Laravle need a log middleware

Packaging program

The execution order of middleware is determined by the packaging function (pack_middleware). The closure returned here is actually equivalent to:

$run = $log($filter($auth($application)));
$run('Laravle');

Writing specifications

Middleware must meet certain specifications: always return a closure, and always pass in the same parameters in the closure (determined by the main logic), The closure always returns the execution result of the handler;

If the logic of the middleware is completed before returning the handle return $handler($name), it is a pre-middleware, otherwise it is a post-middleware.

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