PHP Session
Contents
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.
Start a PHP Session
A session is started with the session_start() function.
The code below should be at the very start of every page:
<?php
// Start the session
session_start();
?>
Set PHP Session Variable Values
Session variables are set with the PHP global variable: $_SESSION:
<?php
// Set session variables
$_SESSION["favcolor"] = "green";
$_SESSION["favanimal"] = "cat";
echo "Session variables are set.";
?>
Get PHP Session Variable Values
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 using ‘session_start()’. This should be at the very start of the page.
Also notice that all session variable values are stored in the global $_SESSION variable. You retrieve a session value using the name given:
<?php
session_start();
?>
<!DOCTYPE html>
<html>
<body>
<?php
// Echo session variables that were set on previous page
echo "Favorite color is " . $_SESSION["favcolor"] . ".<br>";
echo "Favorite animal is " . $_SESSION["favanimal"] . ".";
?>
</body>
</html>
Another way to show all the session variable values for a user session is to run the following code:
<?php
session_start();
?>
<!DOCTYPE html>
<html>
<body>
<?php
print_r($_SESSION);
?>
</body>
</html>
Modify a PHP Session Variable
To change a session variable, just overwrite it:
<?php
session_start();
?>
<!DOCTYPE html>
<html>
<body>
<?php
// to change a session variable, just overwrite it
$_SESSION["favcolor"] = "yellow";
print_r($_SESSION);
?>
</body>
</html>
Destroy a PHP Session
To remove all global session variables and destroy the session, use session_unset() and session_destroy():
<?php
session_start();
?>
<!DOCTYPE html>
<html>
<body>
<?php
// remove all session variables
session_unset();
// destroy the session
session_destroy();
?>
</body>
</html>
Example
You could add the following to the start of every page of your php to control access to your pages:
<?php
session_start();
if (isset(S_POST["logout"])) {
// remove all session variables
session_unset();
// destroy the session
session_destroy();
}
if (!isset($_SESSION["UserID"])) {
echo "User not logged in, redirect to login page";
header ("Refresh:3; url:login.php");
}
else {
echo "<h1>Welcome ".$_SESSION["UserID"]." to the site</h1>";
}
?>
<!DOCTYPE html>
<html>
<body>
<form method=post>
<input type="submit" name="logout" value="logout"/>
</form>
</body>
</html>
Your login.php could set the UserID if the login is successful:
$_SESSION["UserID"] = $_POST["username"];
