The error messages clears if you add the following line to apache2.conf :

ServerName localhost

Now apache2 starts nice and silently. The thing is (and i know how crazy this sounds) the server only answers to https requests so https://localhost works fine (you have to accept all those certificates before it does) but http://localhost doesn't. And even more: in php.ini the line that loads mysql module is commented and with a typo it sez msql.so where it should be mysql.so

I hope this is helpfull to people with the same problem (the https issue makes me wonder though)