My php.ini file is located in /etc/php5/cli.
Actually I have another php.ini file in /etc/php4, so I suppose I have php4 and php5 installed?? Anyway phpBB says my php version is higher than 4.3.3, so he's probably using and reading php5 files, including /etc/php5/cli, am I right?
I've installed php5-mysql and nothing seemed to happen, and yes I have restarted apache after installing that package.
I read something in phpfreaks.com about php5 not having MySQL support enabled by default, so we have to enable mysql_extension(). I just didn't understand how to do it, but it is probably done in the php.ini file. I am still going to explore my php.ini file, because I was out all the day and I didn't have enough time to see it with attention.
Thanks for your anwers. I'll be in touch soon about my latest results