Nowadays, databases are cardinal components of any web based application by enabling websites to provide varying dynamic content. Since very sensitive or secret information can be stored in a database, you should strongly consider protecting your databases.
To retrieve or to store any information you need to connect to the database, send a legitimate query, fetch the result, and close the connection. Nowadays, the commonly used query language in this interaction is the Structured Query Language (SQL). See how an attacker can tamper with an SQL query.
As you can surmise, PHP cannot protect your database by itself. The following sections aim to be an introduction into the very basics of how to access and manipulate databases within PHP scripts.
Keep in mind this simple rule: defense in depth. The more places you take action to increase the protection of your database, the less probability of an attacker succeeding in exposing or abusing any stored information. Good design of the database schema and the application deals with your greatest fears.