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Chapter 7. Storing Information with Databases
The HTML and CSS that give your web site its pretty face reside in individual files on your web server. So does the PHP code that processes forms and performs other dynamic wizardry. There's a third kind of information necessary to a web application, though: data. And while you can store data such as user lists and product information in individual files, most people find it easier to use databases, which are the focus of this chapter.
Lots of information falls under the broad umbrella of "data":
There are three big reasons why this kind of data belongs in a database instead of in files: convenience, simultaneous access, and security. A database program makes it much easier to search for and manipulate individual pieces of information. With a database program, you can do things such as change the email address for user Duck29 to firstname.lastname@example.org in one step. If you put usernames and email addresses in a file, changing an email address would be much more complicated: read the old file, search through each line until you find the one for Duck29, change the line, and write the file back out. If, at same time, one request updates Duck29's email address and another updates the record for user Piggy56, one update could be lost, or (worse) the data file corrupted. Database software manages the intricacies of simultaneous access for you.
In addition to searchability, database programs usually provide you with a different set of access control options compared to files. It is an exacting process to set things up properly so that your PHP programs can create, edit, and delete files on your web server without opening the door to malicious attackers who could abuse that setup to alter your PHP scripts and data files. A database program makes it easier to arrange the appropriate levels of access to your information. It can be configured so that your PHP programs can read and change some information, but only read other information. However the database access control is set up, it doesn't affect how files on the web server are accessed. Just because your PHP program can change values in the database doesn't give an attacker an opportunity to change your PHP programs and HTML files themselves.
The word database is used in a few different ways when talking about web applications. A database can be a pile of structured information, a program (such as MySQL or Oracle) that manages that structured information, or the computer on which that program runs. In this book, I use "database" to mean the pile of structured information. The software that manages the information is a database program, and the computer that the database program runs on is a database server.
Most of this chapter uses the PEAR DB database program abstraction layer. This is an add-on to PHP that simplifies communication between your PHP program and your database program. PEAR (PHP Extension and Application Repository) is a collection of useful modules and libraries for PHP. The DB module is one of the most popular PEAR modules and is bundled with recent versions of PHP. If your PHP installation doesn't have DB installed (Section 7.2, later in this chapter, shows you how to check), see Section A.3 for instructions on how to install it.
When DB isn't available, you need to rely on other PHP functions to talk to your database program. The appropriate set of functions varies with each database program. Some of the more exotic features of your database program may only be accessible through the database-specific functions. Later in this chapter, Section 7.12 discusses shows how to work with the functions in the mysqli extension, which talks to MySQL (Versions 4.1.2 and greater).
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