148 lines
3.2 KiB
Markdown
148 lines
3.2 KiB
Markdown
---
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title: PHP Sessions
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---
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# PHP Sessions
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A session is a way to store information (in variables) to be used across multiple pages.
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Unlike a cookie, the information is not stored on the user's computer.
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## What is a PHP Session?
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When you work with an application, you open it, do some changes, and then you close it. This is much like a Session.
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The computer knows who you are. It knows when you start the application and when you end.
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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_.
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Session variables solve this problem by storing user information to be used across multiple pages (e.g. username, favorite color, etc).
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By default, session variables last until the user closes the browser.
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**Session variables hold information about one single user, and are available to all pages in one application.**
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**Note:** If you need a permanent storage, you may want to store the data in a database.
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## Start a PHP Session
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A session is started with the _session_start()_ function.
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Session variables are set with the PHP global variable: $_SESSION.
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**Example:**
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```
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<?php
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// Start the session
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session_start();
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?>
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<!DOCTYPE html>
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<html>
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<body>
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<?php
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// Set session variables
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$_SESSION["favcolor"] = "blue";
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$_SESSION["favanimal"] = "dog";
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echo "Session variables are set.";
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?>
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</body>
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</html>
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```
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**Note:** The session_start() function must be the **very first thing** in your document. **Before** any HTML tags.
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Output:
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Session variables are set.
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## Get PHP Session Variable Values
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Note 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()).
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Also note that all session variable values are stored in the global $_SESSION variable:
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**Example:**
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```
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<?php
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session_start();
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?>
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<!DOCTYPE html>
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<html>
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<body>
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<?php
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// Echo session variables that were set on previous page
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echo "Favorite color is " . $_SESSION["favcolor"] . ".<br>";
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echo "Favorite animal is " . $_SESSION["favanimal"] . ".";
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?>
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</body>
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</html>
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```
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Output:
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Favorite color is blue.
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Favorite animal is dog.
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Another way to show all the session variable values for a user session is to run the following code:
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```
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<?php
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print_r($_SESSION);
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?>
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```
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### How does it work?
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Most sessions set a user-key on the user's computer that looks something like this: 765487cf34ert8dede5a562e4f3a7e12.
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Then, when a session is opened on another page, it scans the computer for a user-key.
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If there is a match, it accesses that session, if not, it starts a new session.
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## Modify a Session Variable
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To change a session variable, just overwrite it:
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**Example:**
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```
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<?php
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session_start();
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?>
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<!DOCTYPE html>
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<html>
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<body>
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<?php
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// to change a session variable, just overwrite it
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$_SESSION["favcolor"] = "pink";
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print_r($_SESSION);
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?>
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</body>
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</html>
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```
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## Destroy a PHP Session
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To remove all global session variables and destroy the session, use _session_unset()_ and _session_destroy()_:
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**Example:**
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```
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<?php
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session_start();
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?>
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<!DOCTYPE html>
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<html>
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<body>
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<?php
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// remove all session variables
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session_unset();
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// destroy the session
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session_destroy();
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?>
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</body>
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</html>
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```
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