With PHP, you can connect to and manipulate databases.
MySQL is the most popular database system used with PHP.
The data in a MySQL database are stored in tables. A table is a collection of related data, and it consists of columns and rows.
Databases are useful for storing information categorically. A company may have a database with the following tables:
A query is a question or a request.
We can query a database for specific information and have a recordset returned.
Look at the following query (using standard SQL):
SELECT LastName FROM Employees
The query above selects all the data in the "LastName" column from the "Employees" table.
If you don't have a PHP server with a MySQL Database, you can download it for free here: http://www.mysql.com
MySQL is the de-facto standard database system for web sites with HUGE volumes of both data and end-users (like Facebook, Twitter, and Wikipedia).
Another great thing about MySQL is that it can be scaled down to support embedded database applications.
Look at http://www.mysql.com/customers/ for an overview of companies using MySQL.