Laravel middleware for caching routes

CacheOrRetrieve.php

<?php

namespace App\Http\Middleware;

use Closure;
use Illuminate\Support\Facades\Cache;

class CacheOrRetrieve
{
    private $cacheTime = 10;

    /**
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure  $next
     * @return mixed
     */
    public function handle($request, Closure $next)
    {
        $key = $this->cacheKey($request->path());

        if (Cache::has($key)) {
            $content = Cache::get($key);
            $response = response($content);

            if ($this->isJson($content)) {
                $response->header('Content-Type', 'application/json');
            }

            return $response;
        }

        $response = $next($request);

        Cache::put($key, $response->getContent(), $this->cacheTime);

        return $response;
    }

    private function isJson($string) {
        json_decode($string);
        return (json_last_error() == JSON_ERROR_NONE);
    }

    private function cacheKey($path)
    {
        return 'route:' . str_replace('/', '-', $path);
    }
}