PHP- Adding php allow_url_fopen in .conf

I need to add a allow_url_fopen directve for one directory. I’m on Debian 8 Jessie, running PHP 5.6.14. According to PHP doc, setting allow_url_fopen is not allowed in htaccess or in .user.ini so only while editing the .conf

As on Debian, the httpd.conf is named apache2.conf I tried to edit this file under Webmin, to add:
<Directory /home/my_user_name/public_html/my_folder/>
php_flag allow_url_fopen on

But this does nothing. Then I tried in the .conf of my virtual server (etc/apache2/sites-availables/my_domaine.tld) without any better result…

There is also a php5.conf, a fcgi.conf (my PHP is running as CGI/FascCGI) but none of them seems to change the value…

Where can I set the allow_url_fopen on for one directory? in what .conf? Any idea?

Thanks a lot