场景：手头的项目需要对外提供API服务，需要对请求数据进行签名校验。提交接口的数据会有''空字符值存在，laravel的Illuminate\Foundation\Http\Request接收到参数后，会将空字符强制转换为NULL,导致签名校验通不过。

经过一番努力，找到幕后黑手。
Illuminate/Foundation/Http组件中有定义这个中间件。
vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/ConvertEmptyStringsToNull.php

<?php

namespace Illuminate\Foundation\Http\Middleware;

class ConvertEmptyStringsToNull extends TransformsRequest
{
    /**
     * Transform the given value.
     *
     * @param  string  $key
     * @param  mixed  $value
     * @return mixed
     */
    protected function transform($key, $value)
    {
        return is_string($value) && $value === '' ? null : $value;
    }
}

解决方法：
既然是中间件作祟，肯定在内核中有注册。找到内核注释该中间件就好。
app/Http/Kernel.php

class Kernel extends HttpKernel
{
    /**
     * The application's global HTTP middleware stack.
     *
     * These middleware are run during every request to your application.
     *
     * @var array
     */
    protected $middleware = [
        \App\Http\Middleware\CheckForMaintenanceMode::class,
        \Illuminate\Foundation\Http\Middleware\ValidatePostSize::class,
        \App\Http\Middleware\TrimStrings::class,
        //\Illuminate\Foundation\Http\Middleware\ConvertEmptyStringsToNull::class,//会将''转化为null
        \App\Http\Middleware\TrustProxies::class,
        \Illuminate\Session\Middleware\StartSession::class,
    ];