Haversine distance calculation between two points in Laravel

I'm working on a laravel appliciation in which I need to find all the products that are within a certain radius of the user's coordinates. Products have a one to many relationship with users, so users can have multiple products.

I've found that the haversine formula can calculate the distance between two points, but I can't seem to make it work.

I've got the following query in my controller:

$latitude = 51.0258761;
$longitude = 4.4775362;
$radius = 20000;

$products = Product::with('user')
->selectRaw("*,
            ( 6371 * acos( cos( radians(" . $latitude . ") ) *
            cos( radians(user.latitude) ) *
            cos( radians(user.longitude) - radians(" . $longitude . ") ) + 
            sin( radians(" . $latitude . ") ) *
            sin( radians(user.latitude) ) ) ) 
            AS distance")
->having("distance", "<", $radius)
->orderBy("distance")
->get();

I've set the radius to 20000 for testing purposes and it appears all products have a distance of 5687,.. The problem seems to be that the latitude and longitude of the products are stored in the User table, but I'm not sure how I can access those in my query. I've tried user.latitude and 'user->latitude' but nothing seems to work.

Here are my models:

Product model

namespace App;

use IlluminateDatabaseEloquentModel;

class Product extends Model
{
    protected $fillable =
        [
            'soort',
            'hoeveelheid',
            'hoeveelheidSoort',
            'prijsPerStuk',
            'extra',
            'foto',
            'bio'


        ];

    public function User()
    {
        return $this->belongsTo('AppUser');
    }

    public $timestamps = true;
}

User model

namespace App;

use IlluminateAuthAuthenticatable;
use IlluminateDatabaseEloquentModel;
use IlluminateAuthPasswordsCanResetPassword;
use IlluminateFoundationAuthAccessAuthorizable;
use IlluminateContractsAuthAuthenticatable as AuthenticatableContract;
use IlluminateContractsAuthAccessAuthorizable as AuthorizableContract;
use IlluminateContractsAuthCanResetPassword as CanResetPasswordContract;

class User extends Model implements AuthenticatableContract,
                                    AuthorizableContract,
                                    CanResetPasswordContract
{
    use Authenticatable, Authorizable, CanResetPassword;

    protected $table = 'users';

    protected $fillable = 
        [
        'firstName', 
        'lastName', 
        'adres',
        'profilepic',
        'description', 
        'longitude',
        'latitude',
        'email', 
        'password'
    ];

    protected $hidden = ['password', 'remember_token'];

    public function product()
    {
        return $this->hasMany('AppProduct');
    }
}