PHP
Login/logout logic
Learn to build a simple login and logout flow with sessions.
The login flow with sessions
A simple login flow works like this: the username and password are verified. If correct, a $_SESSION['user_id'] or $_SESSION['user'] identifier is written to the session.
On later pages, this identifier is checked with isset($_SESSION['user_id']). If defined, the user is logged in; protected pages are shown.
Never store the password in the session; keep only the user id or role. Password verification happens against the database.
Building the logout flow
Logout is the reverse of login. First remove the user variable with unset($_SESSION['user']). On later pages isset() then returns false.
Then close the whole session with session_destroy(). The session file is deleted; the user is fully logged out.
Tip: a logout.php page runs this code when the logout link is clicked. After logout, redirect to the login page.
Exercise Write a simple login check
Call session_start(). Set $username to 'admin' and $password to 'secret'. With an if, check both: if correct, set $_SESSION['user_id'] = 1 and print ok; in else, print denied.
Show example solution
Call session_start, set $username and $password, check both with === in an if, on success $_SESSION['user_id']=1 and echo ok, else echo denied.
Example solution:
<?php
// Simple login: write user_id to session on correct credentials
session_start();
$username = 'admin';
$password = 'secret';
if ($username === 'admin' && $password === 'secret') {
$_SESSION['user_id'] = 1;
echo "ok\n";
} else {
echo "denied\n";
}
Test yourself
-
What happens on a successful login?
- session_destroy() is called
- A user identifier is written to $_SESSION
- session_id() is reset
- The cookie is deleted
Show answer
A user identifier is written to $_SESSION — Correct. On login, a user identifier is written to $_SESSION; later pages read it.
-
Write to the session on a successful login
<?php
session_start();
if ($username === 'admin' && $password === 'secret') {
____
echo 'login ok';
} else {
echo 'denied';
}
Show answer
<?php
session_start();
if ($username === 'admin' && $password === 'secret') {
$_SESSION['user'] = ['id' => 1];
echo 'login ok';
} else {
echo 'denied';
}
-
Match the login and logout steps
- Login succeeds
- Write $_SESSION['user_id']
- Logout request
- unset + session_destroy
- $_SESSION['user_id']
- Check if logged in
- session_destroy()
- Closes the session fully
Next lesson: Summary - Sessions
