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":
Who your users are, such as their names and email addresses.
What your users do, such as message board posts and profile information.
The "stuff" that your site is about, such as a list of record albums, a product catalog, or what's for dinner.
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
email@example.com 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
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