Laravel returns array of ID's as string instead of integers (CentOS)

Question

dear community

I'm actually building a project using Laravel (5.2) and AngularJS (Server on CentOS - Local/Testing on Windows 7), and i found something strange while requesting an array of Id's as seen on these screenshots: Response on Windows, Response on Linux.

On windows i get an array of integers, and on centos i get an array of strings...

The question is, What could it be causing this, and how to get the same response on centos as on windows?

PS: this is the database

contacts
-----------------
id
name
lastname
phone
....

groups
-------------------
id
nombre

contact_groups
-------------------
contact_id
group_id

and this is the function i used on Model Groups.php

protected $appends = ['contactList'];

public function getContactListAttribute(){
    return $this->contacts()->lists('id');
}