willaengine/app/WmPackage.php

<?php
namespace App;

use Illuminate\Database\Eloquent\Model;

class WmPackage extends Model
{

    public function WmItems()
    {
        return $this->belongsToMany('App\WmItem')->withPivot('quantity','details');
    }

    public function getDetailsAttribute()
    {
        return json_decode($this->attributes['details']);
    }

    public function setDetailsAttribute($value)
    {
        $this->attributes['details'] = json_encode($value, JSON_UNESCAPED_UNICODE);
    }

}