To start a PHP session, you must first call the session_start() function, and it must be placed at the beginning of the script and before any output; secondly, store and retrieve data through the $_SESSION array, pay attention to check whether the variable exists and avoid storing sensitive information; finally, you must manually clear the $_SESSION array and call session_destroy() to delete the session cookies if necessary and redirect the user.
Starting a session in PHP is pretty straightforward, but there are a few key things to get right. The most important part is using the session_start() function — that's what actually kicks off the session. But where and how you use it matters, especially if you're dealing with login systems, shopping carts, or any kind of user tracking.
Where to put session_start()
You need to call session_start() at the very beginning of your script — before any output is sent to the browser. That means no HTML, no echo statements, not even a space or newline before the opening <?php tag.
If you try to start a session after sending output, PHP will throw an error like:
Warning: Cannot send session cache limiter - headers already sent
So always place session_start() at the top of your PHP file, like this:
<?php session_start(); // rest of your code here ?>
One exception: if you're including or requiring other files, make sure none of them generate output before session_start() runs.
How to store and retrieve session data
Once the session is started, you can store data in the $_SESSION superglobal array. This data persists across pages as long as the session is active.
For example, saving a username:
$_SESSION['username'] = 'john_doe';
Then on another page (as long as you call session_start() again), you can access it like:
echo 'Welcome back, ' . $_SESSION['username'];
Some common uses:
- Storing user IDs after login
- Keeping track of form inputs between steps
- Saving preferences temporary
A couple of things to remember:
- Only store necessary data — sessions can get heavy if overused
- Always check if a session variable exists before using it, like with
isset($_SESSION['key']) - Avoid storing sensitive information like passwords (use tokens or hashes instead)
Ending or destroying a session
Ending a session isn't just about closing the browser. If you want to log someone out or clear session data, you need to do a few things manually.
To destroy all session data:
session_start(); $_SESSION = array(); // clear all session variables session_destroy(); // end the session
Also, if you want to remove just one item:
unset($_SESSION['username']);
Keep in mind:
-
session_destroy()only removes the data on the server — it doesn't automatically unset the session cookie - To fully log someone out, you may also want to delete the session cookie
- Always redirect the user after logging them out to avoid confusion
Starting a session in PHP really comes down to three main points: calling session_start() early, managing $_SESSION carefully, and cleaning up when needed. It's simple enough once you know where to place things, but easy to mess up if you skip a step.
Basically that's it.
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