Programming PHP

A third technique for maintaining state is to use cookies. A cookie is a bit of information
that the server can give to a client. On every subsequent request the client will
give that information back to the server, thus identifying itself. Cookies are useful for
retaining information through repeated visits by a browser, but they’re not without
their own problems. The main problem is that some browsers don’t support cookies,
and even with browsers that do, the user can disable cookies. So any application
that uses cookies for state maintenance needs to use another technique as a fallback
mechanism. We’ll discuss cookies in more detail shortly.
The best way to maintain state with PHP is to use the built-in session-tracking system.
This system lets you create persistent variables that are accessible from different
pages of your application, as well as in different visits to the site by the same user.
Behind the scenes, PHP’s session-tracking mechanism uses cookies (or URLs) to elegantly
solve most problems that require state, taking care of all the details for you.
We’ll cover PHP’s session-tracking system in detail later in this chapter.
Cookies
A cookie is basically a string that contains several fields. A server can send one or
more cookies to a browser in the headers of a response. Some of the cookie’s fields
indicate the pages for which the browser should send the cookie as part of the
request. The value field of the cookie is the payload—servers can store any data
they like there (within limits), such as a unique code identifying the user, preferences,
etc.
Use the setcookie() function to send a cookie to the browser:
setcookie(name [, value [, expire [, path [, domain [, secure ]]]]]);
This function creates the cookie string from the given arguments and creates a
Cookie header with that string as its value. Because cookies are sent as headers in the
response, setcookie() must be called before any of the body of the document is sent.
The parameters of setcookie() are:
name
A unique name for a particular cookie. You can have multiple cookies with different
names and attributes. The name must not contain whitespace or semicolons.
value
The arbitrary string value attached to this cookie. The original Netscape specification
limited the total size of a cookie (including name, expiration date, and
other information) to 4 KB, so while there’s no specific limit on the size of a
cookie value, it probably can’t be much larger than 3.5 KB.
expire
The expiration date for this cookie. If no expiration date is specified, the
browser saves the cookie in memory and not on disk. When the browser exits,
the cookie disappears. The expiration date is specified as the number of seconds
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Maintaining State | 179