PHP manages sessions and cookies to maintain state across HTTP requests. 1. Sessions store data server-side, using a unique session ID stored typically in a cookie (PHPSESSID). 2. Cookies store data client-side, set via setcookie() and accessed through $_COOKIE. 3. Sessions are safer for sensitive or large data, while cookies suit non-sensitive, persistent data. 4. Security practices include regenerating session IDs, setting secure cookie flags, and avoiding storing secrets directly. Example usage includes session_start() and setcookie() with appropriate parameters.
PHP handles sessions and cookies in different but related ways to help maintain state between HTTP requests, which are otherwise stateless. Sessions store data on the server, while cookies store data on the client side. Both are commonly used for things like user authentication, tracking preferences, or maintaining shopping carts.

Here’s how PHP works with each of them and what you should keep in mind when using them.
How PHP manages sessions
When a session starts using session_start()
, PHP generates a unique identifier (called a session ID) and stores session data on the server. The session ID is usually sent to the browser via a cookie (PHPSESSID
by default), so the browser can send it back on subsequent requests.

- Session data lives on the server, typically in files unless configured otherwise.
- You access session data through the superglobal
$_SESSION
. - Sessions last until the browser is closed or until they time out, depending on configuration.
A few important notes:
- Always call
session_start()
before accessing or setting any session data. - Don’t forget to close sessions properly if needed (though PHP does this automatically).
- Avoid storing sensitive data directly in sessions — instead, store references (like a user ID), not raw secrets.
Example:

session_start(); $_SESSION['username'] = 'johndoe';
This will save the username in the session, accessible across pages as long as the session is active.
Working with cookies in PHP
Cookies are small pieces of data stored in the user’s browser. In PHP, you set cookies using the setcookie()
function before any output is sent (since it modifies HTTP headers).
- Cookies can be accessed via the
$_COOKIE
superglobal. - They can be persistent (saved beyond the current session) or temporary (deleted when the browser closes).
- You can control their lifetime, path, domain, and security settings.
Some key points:
- Be careful with expiration times — use
time() 3600
style timestamps. - Set secure and httponly flags for better cookie security.
- Never trust cookie data blindly — treat it like user input.
Example:
setcookie('theme', 'dark', time() 86400, '/', '', true, true);
This sets a secure, HTTP-only cookie that lasts a day and is available site-wide.
Session vs. Cookie: When to use which
Understanding when to use sessions and when to use cookies helps avoid misuse and potential security issues.
- Use sessions when you need to store sensitive or large amounts of data on the server. Since only the session ID is exposed to the client, it’s safer than cookies.
-
Use cookies for non-sensitive data that needs to persist across visits or be accessible to JavaScript (unless you set
httponly
). - Combine both when needed — for example, storing a session ID in a cookie and keeping actual data server-side.
Also, keep in mind:
- Sessions rely on cookies by default to pass the session ID.
- Cookies have size limits (usually around 4KB per domain).
- Sessions can be configured to use other storage mechanisms like databases or Redis.
Security considerations
Both sessions and cookies can introduce vulnerabilities if not handled carefully.
- Regenerate session IDs after login using
session_regenerate_id(true)
to prevent fixation attacks. - Set cookie options securely — always use
Secure
(HTTPS only) andHttpOnly
where possible. - Limit cookie scope by setting appropriate
path
anddomain
.
You don’t need to overdo it every time, but being aware of these points makes your app more robust.
That's basically how PHP works with sessions and cookies. It’s not complicated, but there are enough moving parts to make it worth understanding clearly.
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