现在，为了使得网站能够提供各种各样动态的内容，数据库已经成为所有基于 WEB 应用程序最重要的组件。由于一些十分敏感或者保密的信息可能会存储在这样的数据库中，因此，您需要非常慎重的考虑如何保护它们。
为了能够存储或者检索信息，您需要连接到数据库，发送一个合法的查询命令，得到结果，然后关闭连接。目前，在这个交互过程中最常用的查询语言是“结构化查询语言（Structured Query Language, SQL）”。请参阅以下内容以了解黑客如何“利用 SQL 查询攻击”。
正如您所知道的，PHP 自身并无法保护您的数据库。以下章节的内容旨在介绍有关如何利用 PHP 脚本访问和操作数据库的最基础的知识。
您最好不要把所有的事物过程都放在 WEB 应用程序（即您的脚本）中来实施，而充分的利用数据库结构，使用视图（view）、触发器（trigger）或者规则（rule）。如果需要对系统进行升级，则需要对数据库开辟新的端口，因此您需要对每一个独立的数据库客户重新设置权限。总之，触发器可以被用来透明地和自动地处理字段，该特性可以在调试应用程序地问题或者追踪后台事物时提供便利信息。
您可能希望通过 SSL 建立连接来加密客户端和服务端之间的通讯以增加安全性，或者您可以使用 ssh 来加密客户端和数据库服务器之间的网络连接。如果您实施了这些措施，则监视您的网络流量以及以这种方式获取信息将变成十分复杂的工作。
SSL/SSH protects data travelling from the client to the server, SSL/SSH does not protect the persistent data stored in a database. SSL is an on-the-wire protocol.
Once an attacker gains access to your database directly (bypassing the webserver), the stored sensitive data may be exposed or misused, unless the information is protected by the database itself. Encrypting the data is a good way to mitigate this threat, but very few databases offer this type of data encryption.
The easiest way to work around this problem is to first create your own encryption package, and then use it from within your PHP scripts. PHP can assist you in this case with its several extensions, such as Mcrypt and Mhash, covering a wide variety of encryption algorithms. The script encrypts the data be stored first, and decrypts it when retrieving. See the references for further examples how encryption works.
In case of truly hidden data, if its raw representation is not needed (i.e. not be displayed), hashing may be also taken into consideration. The well-known example for the hashing is storing the MD5 hash of a password in a database, instead of the password itself. See also crypt() and md5().
Many web developers are unaware of how SQL queries can be tampered with, and assume that an SQL query is a trusted command. It means that SQL queries are able to circumvent access controls, thereby bypassing standard authentication and authorization checks, and sometimes SQL queries even may allow access to host operating system level commands.
Direct SQL Command Injection is a technique where an attacker creates or alters existing SQL commands to expose hidden data, or to override valuable ones, or even to execute dangerous system level commands on the database host. This is accomplished by the application taking user input and combining it with static parameters to build a SQL query. The following examples are based on true stories, unfortunately.
Owing to the lack of input validation and connecting to the database on behalf of a superuser or the one who can create users, the attacker may create a superuser in your database.
If it happened, then the script would present a superuser access to him. Note that 0; is to supply a valid offset to the original query and to terminate it.
A feasible way to gain passwords is to circumvent your search result pages. What the attacker needs only is to try if there is any submitted variable used in SQL statement which is not handled properly. These filters can be set commonly in a preceding form to customize WHERE, ORDER BY, LIMIT and OFFSET clauses in SELECT statements. If your database supports the UNION construct, the attacker may try to append an entire query to the original one to list passwords from an arbitrary table. Using encrypted password fields is strongly encouraged.
SQL UPDATEs are also subject to attacking your database. These queries are also threatened by chopping and appending an entirely new query to it. But the attacker might fiddle with the SET clause. In this case some schema information must be possessed to manipulate the query successfully. This can be acquired by examing the form variable names, or just simply brute forcing. There are not so many naming convention for fields storing passwords or usernames.
A frightening example how operating system level commands can be accessed on some database hosts.
MSSQL Server executes the SQL statements in the batch including a command to add a new user to the local accounts database. If this application were running as sa and the MSSQLSERVER service is running with sufficient privileges, the attacker would now have an account with which to access this machine.
You may plead that the attacker must possess a piece of information about the database schema in most examples. You are right, but you never know when and how it can be taken out, and if it happens, your database may be exposed. If you are using an open source, or publicly available database handling package, which may belong to a content management system or forum, the intruders easily produce a copy of a piece of your code. It may be also a security risk if it is a poorly designed one.
These attacks are mainly based on exploiting the code not being written with security in mind. Never trust on any kind of input, especially which comes from the client side, even though it comes from a select box, a hidden input field or a cookie. The first example shows that such a blameless query can cause disasters.
Besides these, you benefit from logging queries either within your script or by the database itself, if it supports. Obviously, the logging is unable to prevent any harmful attempt, but it can be helpful to trace back which application has been circumvented. The log is not useful by itself, but through the information it contains. The more detail is generally better.