$prop = "myProp";
$user->$prop->toArray();

namespace App\Models;

use Permafrost\DynamicRelations\DynamicModel;

class User extends DynamicModel
{
    public static $dynamicRelations = [
      'abc', 'def',
    ];
    
    public function abc()
    {
        return $this->hasMany('App\Models\Abc');
    }
    
    public function def()
    {
        return $this->hasMany('App\Models\Def');
    }
    
}

$user = User::find(1);
$prop = "abc";
$user->$prop->toArray();