A session is a way to store information [in variables] to be used across multiple pages.
Unlike a cookie, the information is not stored on the users computer.
What is a PHP Session?
When you work with an application, you open it, do some changes, and then you close it. This is much like a Session. The computer knows who you are. It knows when you start the application and when you end. But on the internet there is one problem: the web server does not know who you are or what you do, because the HTTP address doesn't maintain state.
Session variables solve this problem by storing user information to be used across multiple pages [e.g. username, favorite color, etc]. By default, session variables last until the user closes the browser.
So; Session variables hold information about one single user, and are available to all pages in one application.
Tip: If you need a permanent storage, you may want to store the data in a database.
Start a PHP Session
A session is started with the session_start[] function.
Session variables are set with the PHP global variable: $_SESSION.
Now, let's create a new page called "demo_session1.php". In this page, we start a new PHP session and set some session variables:
Example
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Note: The session_start[] function must be the very first thing in your document. Before any HTML tags.
Get PHP Session Variable Values
Next, we create another page called "demo_session2.php". From this page, we will access the session information we set on the first page ["demo_session1.php"].
Notice that session variables are not passed individually to each new page, instead they are retrieved from the session we open at the beginning of each page [session_start[]].
Also notice that all session variable values are stored in the global $_SESSION variable:
Example
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Another way to show all the session variable values for a user session is to run the following code:
Example
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How does it work? How does it know it's me?
Most sessions set a user-key on the user's computer that looks something like this: 765487cf34ert8dede5a562e4f3a7e12. Then, when a session is opened on another page, it scans the computer for a user-key. If there is a match, it accesses that session, if not, it starts a new session.
Modify a PHP Session Variable
To change a session variable, just overwrite it:
Example
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Destroy a PHP Session
To remove all global session variables and destroy the session, use session_unset[] and session_destroy[]:
Example
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PHP Exercises
In the general situation :
- the session id is sent to the user when his session is created.
- it is stored in a cookie [called, by default,
PHPSESSID] - that cookie is sent by the browser to the server with each request
- the server [PHP] uses that cookie, containing the session_id, to know which file corresponds to that user.
The data in the sessions files is the content of $_SESSION, serialized [ie, represented as a string --
with a function such as serialize] ; and is un-serialized when the file is loaded by PHP, to populate the $_SESSION array.
Sometimes, the session id is not stored in a cookie, but sent in URLs, too -- but that's quite rare, nowadays.
For more informations, you can take a look at the Session Handling section of
the manual, that gives some useful informations.
For instance, there is a page about Passing the Session ID, which explains how the session id is passed from page to page, using a cookie, or in URLs -- and which configuration options affect this.