Display cms page in a module page - prestashop-1.7

Question

I use a 3rd party module for the Customer Service of my website.
In this module, there is a dedicate page to create a ticket.
I try to add a cms page (already create in Pages of Prestashop) in the left of the form of this page.
For that, I create a function in modules/{THEmodule}/controllers/front/function.php
public function getFAQ($id_cms, $id_lang = null, $id_shop = null){
if (is_null($id_lang)) {
$id_lang = (int)Configuration::get('PS_LANG_DEFAULT');
}
if (is_null($id_shop)) {
$id_shop = (int)Configuration::get('PS_SHOP_DEFAULT');
}
$sql = new DbFAQ();
$sql->select('content');
$sql->from('cms_lang');
$sql->where('id_cms = .(int)$id_cms.' AND 'id_lang = .(int)$id_lang.' AND 'id_shop = .(int)$id_shop');
return Db::getInstance()->executeS($sql);
}
Then I call the function in .tpl
<div id="support-getFAQ">
{$getFAQ=12} {* 12 is the id of the cms page that I want display *}
</div>
But when I check the page, nothing is display, so I guess is not the good way.
Could someone help me?

Answer 1

Thanks to #marcin-jaworski to give me the way.
The solution is simple.
No need to add function, just write in tpl:
{assign var=new_smarty_var value=CMS::getCMSContent(12)}
{$new_smarty_var.content nofilter}
Don't forget "nofilter" to print html.

Answer 2

Because of good practise you should assign smarty variable in your module controller or display function for hook.
PrestaShop DevDocs
Displaying content on the front office
You can get CMS content by use function already made in CMS class (/classes/CMS.php):
public static function getCMSContent($id_cms, $id_lang = null, $id_shop = null)
public static function getCMSPages($id_lang = null, $id_cms_category = null, $active = true, $id_shop = null)
Also i have little trick witch you can use in .tpl
{assign var=new_smarty_var value=(CMS::getOneCMS(11, $language.id)}
{$new_smarty_var.content}