Follow the guide and install the elements one by one! I tried to install everything in a row and it screwed everything.

APACHE2:
This will install and configure APACHE. The root folder is /var/www..

# apt-get install apache2

PHP5:
This will install and configure PHP5 to work with APACHE2.

# apt-get install php5

To check if the interaction between PHP and APACHE is functional, create /var/www/essay.php containing:

<?php
    phpinfo();
?>

Then launch it from your explorer from http://localhost/essay.php
To configure PHP:
PHP is modular, several module exist and can be loaded or not in /etc/php.ini

A list of available modules for PHP is at http://www.php.net, and the debian package corresponding are:

$ apt-cache search php5 | less

MySQL:

# apt-get install mysql-server

Configuring MySQL:
One need to know that UNIX and MySQL users are different. We start by typing in a root terminal:

mysql

We should then get something like:

Welcome to the MySQL monitor.  Commands end with ; or g. 
Your MySQL connection id is 133 to server version: 3.23.49-log
Type 'help;' or 'h' for help. Type 'c' to clear the buffer.
mysql>

The SQL connection to the server works properly. Now exit :

mysql> quit 
Bye

We need to give a password to the root user for Mysql :

mysqladmin password yourpassword

Make a root connection to the SQL server :

mysql -p
Enter password : 
Welcome to the MySQL monitor.  Commands end with ; or g.
Your MySQL connection id is 9 to server version: 5.0.20-Debian_1-log

Type 'help;' or 'h' for help. Type 'c' to clear the buffer.

mysql>

To prevent using root for the connections, we now create a adminsql user:

mysql> use mysql;  
mysql> select * from user;
mysql>  GRANT ALL PRIVILEGES ON *.* TO adminmysql@localhost IDENTIFIED BY 'motdepasse' WITH GRANT OPTION;
mysql>  FLUSH PRIVILEGES;
mysql> select * from user where User='adminmysql';
mysql> quit
Bye

Now we connect like:

mysql -u adminmysql -p

That's it for the first MySQL configuration

To check the SQL server :

ps aux |grep mysql

The porcess is called mysqld.

/etc/init.d/mysql stop // to stop the service
/etc/init.d/mysql start // to start the service

To insure a good interaction between PHP5 et mysql :

apt-get install php5-mysql

PhpMyAdmin:
PhpMyAdmin is not mandatory but it's a great help to manage databases!

apt-get install phpmyadmin

To check that everything is functional, go to : http://localhost/phpmyadmin/
If you followed the previous steps, you should get the PhpMyAdmin main page, asking for a password.
This is the one you set previously with adminmysql user.

Everything should be functional, you manage your database with phpmyadmin and you put your files to /var/www.
You can of course change the root document in /etc/apache2/httpd.conf