Let's start with installing apache essentials:
sudo apt-get install apache2 apache2-doc apache2-utils
then PHP5:
sudo apt-get install libapache2-mod-php5 php5 php-pear php5-mysql php5-xcache
and finally MariaDB... but wait, it's nowhere to be found in official raspbmc repository, so since I had some problems adding it and installing from official MariaDB repo, I used this one instead. So install it by simply doing:
sudo apt-get install mariadb-server
During MariaDB installation you should be prompted to set up root password.
Frankly.... I installed this some time ago and didn't take proper notes :( Well, maybe next time I install the RAMP stack I will work on this post ;) Sorry