先看看5.4手册中关于fetch模式的一段简述：

Laravel no longer includes the ability to customize the PDO fetch mode from your configuration files. Instead, PDO::FETCH_OBJ is always used. If you would still like to customize the fetch mode for your application you may listen for the new Illuminate\Database\Events\StatementPrepared event:

Event::listen(StatementPrepared::class, function ($event) {
 $event->statement->setFetchMode(...);
});

大概意思也就是说从5.4开始， 不能再通过配置文件的方式去改变fetch mode了。而使用了事件监听设置去实现该配置。

解决方案：

打开app/Providers/EventServiceProvier.php， 按照官方文档，先引入IlluminateDatabaseEventsStatementPrepared类，然后在boot方法中加入上方样例代码，最终代码如下：

<?php
namespace App\Providers;

use Illuminate\Support\Facades\Event;
use Illuminate\Foundation\Support\Providers\EventServiceProvider as ServiceProvider;
use Illuminate\Database\Events\StatementPrepared;

class EventServiceProvider extends ServiceProvider
{
    /**
     * The event listener mappings for the application.
     *
     * @var array
     */
    protected $listen = [
        'App\Events\Event' => [
            'App\Listeners\EventListener',
        ],
    ];

    /**
     * Register any events for your application.
     *
     * @return void
     */
    public function boot()
    {
        parent::boot();

        //
        Event::listen(StatementPrepared::class, function ($event) {
            $event->statement->setFetchMode(\PDO::FETCH_ASSOC);
        });
    }
}

楼主需要先按照手册设置这么个大前提，然后再配合toArray方法，即可达到你想要的结果。