Laravel 5.5 API versioning based header. It's not working because the route has already been binded, but I kept it here for the record

ApiVersion.php

<?php namespace App;

/**
 * ApiVersion class
 *
 * @author    pvl
 */
class ApiVersion
{
    private static $valid_api_versions = [
        1 => 'v1',
        2 => 'v2',
    ];

    /**
     * Resolve the requested api version.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return integer
     */
    public static function get($request) {
        return (int) $request->header('api-version');
    }

    /**
     * Determines if a version is valid or not
     *
     * @param integer $apiVersion
     * @return bool
     */
    public static function isValid($apiVersion) {
        return array_key_exists($apiVersion, self::$valid_api_versions);
    }

    /**
     * Resolve namespace for a api version
     *
     * @param integer $apiVersion
     * @return string
     */
    public static function getNamespace($apiVersion)
    {
        if (!self::isValid($apiVersion)) {
            return null;
        }

        return self::$valid_api_versions[$apiVersion];
    }
}

ApiVersionMiddleware.php

<?php namespace App\Http\Middleware;

use App\ApiVersion;
use Closure;
use Illuminate\Http\Request;
use Symfony\Component\HttpKernel\Exception\BadRequestHttpException;

/**
 * ApiVersionMiddleware class
 *
 * @author    pvl
 */
class ApiVersionMiddleware
{

    public function handle(Request $request, Closure $next)
    {
        $route = $request->route();
        $actions = $route->getAction();

        $requestedApiVersion = ApiVersion::get($request);
        if (!ApiVersion::isValid($requestedApiVersion)) {
            throw new BadRequestHttpException('No valid api version given');
        }

        $apiNamespace = ApiVersion::getNamespace($requestedApiVersion);
        $actions['uses'] = str_replace('v1', $apiNamespace, $actions['uses']);
        $actions['namespace'] .= "\\" . $apiNamespace;
        $actions['controller'] = str_replace('v1', $apiNamespace, $actions['controller']);

        $route->setAction($actions);

        return $next($request);
    }
}

Kernel.php

<?php

namespace App\Http;

use Illuminate\Foundation\Http\Kernel as HttpKernel;

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 = [
        \Illuminate\Foundation\Http\Middleware\CheckForMaintenanceMode::class,
        \Illuminate\Foundation\Http\Middleware\ValidatePostSize::class,
        \App\Http\Middleware\TrimStrings::class,
        \Illuminate\Foundation\Http\Middleware\ConvertEmptyStringsToNull::class,
        \App\Http\Middleware\TrustProxies::class,
    ];

    /**
     * The application's route middleware groups.
     *
     * @var array
     */
    protected $middlewareGroups = [
        'web' => [
            \App\Http\Middleware\EncryptCookies::class,
            \Illuminate\Cookie\Middleware\AddQueuedCookiesToResponse::class,
            \Illuminate\Session\Middleware\StartSession::class,
            // \Illuminate\Session\Middleware\AuthenticateSession::class,
            \Illuminate\View\Middleware\ShareErrorsFromSession::class,
            \App\Http\Middleware\VerifyCsrfToken::class,
            \Illuminate\Routing\Middleware\SubstituteBindings::class,
        ],

        'api' => [
            'api-version',
            'throttle:60,1',
            'bindings',
        ],
    ];

    /**
     * The application's route middleware.
     *
     * These middleware may be assigned to groups or used individually.
     *
     * @var array
     */
    protected $routeMiddleware = [
        'auth' => \Illuminate\Auth\Middleware\Authenticate::class,
        'auth.basic' => \Illuminate\Auth\Middleware\AuthenticateWithBasicAuth::class,
        'bindings' => \Illuminate\Routing\Middleware\SubstituteBindings::class,
        'can' => \Illuminate\Auth\Middleware\Authorize::class,
        'guest' => \App\Http\Middleware\RedirectIfAuthenticated::class,
        'throttle' => \Illuminate\Routing\Middleware\ThrottleRequests::class,
        'api-version' => \App\Http\Middleware\ApiVersionMiddleware::class,
    ];
}

route_api.php

<?php

use Illuminate\Http\Request;

/*
|--------------------------------------------------------------------------
| API Routes
|--------------------------------------------------------------------------
|
| Here is where you can register API routes for your application. These
| routes are loaded by the RouteServiceProvider within a group which
| is assigned the "api" middleware group. Enjoy building your API!
|
*/

Route::middleware('api')->get('/user', 'v1\UserController@index');