Programming PHP

Session basics
To enable sessions for a page, call session_start() before any of the document has
been generated:
...
This assigns a new session ID if it has to, possibly creating a cookie to be sent to the
browser, and loads any persistent variables from the store.
If you have registered objects, the class definitions for those objects must be loaded
before the call to session_start(). See Chapter 6 for discussion and an example.
You can register a variable with the session by passing the name of the variable to
session_register(). For example, here is a basic hit counter:
This page has been viewed times.
The session_start() function loads registered variables into the associative array
$HTTP_SESSION_VARS. The keys are the variables’ names (e.g., $HTTP_SESSION_
VARS['hits']). If register_globals is enabled in the php.ini file, the variables are also
set directly. Because the array and the variable both reference the same value, setting
the value of one also changes the value of the other.
You can unregister a variable from a session, which removes it from the data store,
by calling session_unregister(). The session_is_registered() function returns true
if the given variable is registered. If you’re curious, the session_id() function returns
the current session ID.
To end a session, call session_destroy(). This removes the data store for the current
session, but it doesn’t remove the cookie from the browser cache. This means that,
on subsequent visits to sessions-enabled pages, the user will have the same session
ID she had before the call to session_destroy(), but none of the data.
Example 7-13 shows the first code block from Example 7-11 rewritten to use sessions
instead of manually setting cookies.
Example 7-13. Setting preferences with sessions