How do I expire a PHP session after 30 minutes?

You should implement a session timeout of your own. Both options mentioned by others (session.gc_maxlifetime and session.cookie_lifetime) are not reliable. I'll explain the reasons for that.

First:

session.gc_maxlifetime
session.gc_maxlifetime specifies the number of seconds after which data will be seen as 'garbage' and cleaned up. Garbage collection occurs during session start.

But the garbage collector is only started with a probability of session.gc_probability divided by session.gc_divisor. And using the default values for those options (1 and 100 respectively), the chance is only at 1%.

Well, you could simply adjust these values so that the garbage collector is started more often. But when the garbage collector is started, it will check the validity for every registered session. And that is cost-intensive.

Furthermore, when using PHP's default session.save_handler files, the session data is stored in files in a path specified in session.save_path. With that session handler, the age of the session data is calculated on the file's last modification date and not the last access date:

Note: If you are using the default file-based session handler, your filesystem must keep track of access times (atime). Windows FAT does not so you will have to come up with another way to handle garbage collecting your session if you are stuck with a FAT filesystem or any other filesystem where atime tracking is not available. Since PHP 4.2.3 it has used mtime (modified date) instead of atime. So, you won't have problems with filesystems where atime tracking is not available.

So it additionally might occur that a session data file is deleted while the session itself is still considered as valid because the session data was not updated recently.

And second:

session.cookie_lifetime
session.cookie_lifetime specifies the lifetime of the cookie in seconds which is sent to the browser. […]

Yes, that's right. This only affects the cookie lifetime and the session itself may still be valid. But it's the server's task to invalidate a session, not the client. So this doesn't help anything. In fact, having session.cookie_lifetime set to 0 would make the session’s cookie a real session cookie that is only valid until the browser is closed.

Conclusion / best solution:

The best solution is to implement a session timeout of your own. Use a simple time stamp that denotes the time of the last activity (i.e. request) and update it with every request:

if (isset($_SESSION['LAST_ACTIVITY']) && (time() - $_SESSION['LAST_ACTIVITY'] > 1800)) {
    // last request was more than 30 minutes ago
    session_unset();     // unset $_SESSION variable for the run-time 
    session_destroy();   // destroy session data in storage
}
$_SESSION['LAST_ACTIVITY'] = time(); // update last activity time stamp

Updating the session data with every request also changes the session file's modification date so that the session is not removed by the garbage collector prematurely.

You can also use an additional time stamp to regenerate the session ID periodically to avoid attacks on sessions like session fixation:

if (!isset($_SESSION['CREATED'])) {
    $_SESSION['CREATED'] = time();
} else if (time() - $_SESSION['CREATED'] > 1800) {
    // session started more than 30 minutes ago
    session_regenerate_id(true);    // change session ID for the current session and invalidate old session ID
    $_SESSION['CREATED'] = time();  // update creation time
}

Notes:

• session.gc_maxlifetime should be at least equal to the lifetime of this custom expiration handler (1800 in this example);
• if you want to expire the session after 30 minutes of activity instead of after 30 minutes since start, you'll also need to use setcookie with an expire of time()+60*30 to keep the session cookie active.

Simple way of PHP session expiry in 30 minutes.

Note : if you want to change the time, just change the 30 with your desired time and do not change * 60: this will gives the minutes.

In minutes : (30 * 60)
In days : (n * 24 * 60 * 60 ) n = no of days

Login.php

<?php
    session_start();
?>

<html>
    <form name="form1" method="post">
        <table>
            <tr>
                <td>Username</td>
                <td><input type="text" name="text"></td>
            </tr>
            <tr>
                <td>Password</td>
                <td><input type="password" name="pwd"></td>
            </tr>
            <tr>
                <td><input type="submit" value="SignIn" name="submit"></td>
            </tr>
        </table>
    </form>
</html>

<?php
    if (isset($_POST['submit'])) {
        $v1 = "FirstUser";
        $v2 = "MyPassword";
        $v3 = $_POST['text'];
        $v4 = $_POST['pwd'];
        if ($v1 == $v3 && $v2 == $v4) {
            $_SESSION['luser'] = $v1;
            $_SESSION['start'] = time(); // Taking now logged in time.
            // Ending a session in 30 minutes from the starting time.
            $_SESSION['expire'] = $_SESSION['start'] + (30 * 60);
            header('Location: http://localhost/somefolder/homepage.php');
        } else {
            echo "Please enter the username or password again!";
        }
    }
?>

HomePage.php

<?php
    session_start();

    if (!isset($_SESSION['luser'])) {
        echo "Please Login again";
        echo "<a href='http://localhost/somefolder/login.php'>Click Here to Login</a>";
    }
    else {
        $now = time(); // Checking the time now when home page starts.

        if ($now > $_SESSION['expire']) {
            session_destroy();
            echo "Your session has expired! <a href='http://localhost/somefolder/login.php'>Login here</a>";
        }
        else { //Starting this else one [else1]
?>
            <!-- From here all HTML coding can be done -->
            <html>
                Welcome
                <?php
                    echo $_SESSION['luser'];
                    echo "<a href='http://localhost/somefolder/logout.php'>Log out</a>";
                ?>
            </html>
<?php
        }
    }
?>

LogOut.php

<?php
    session_start();
    session_destroy();
    header('Location: http://localhost/somefolder/login.php');
?>

Is this to log the user out after a set time? Setting the session creation time (or an expiry time) when it is registered, and then checking that on each page load could handle that.

E.g.:

$_SESSION['example'] = array('foo' => 'bar', 'registered' => time());

// later

if ((time() - $_SESSION['example']['registered']) > (60 * 30)) {
    unset($_SESSION['example']);
}

Edit: I've got a feeling you mean something else though.

You can scrap sessions after a certain lifespan by using the session.gc_maxlifetime ini setting:

Edit: ini_set('session.gc_maxlifetime', 60*30);

This post shows a couple of ways of controlling the session timeout: http://bytes.com/topic/php/insights/889606-setting-timeout-php-sessions

IMHO the second option is a nice solution:

<?php
/***
 * Starts a session with a specific timeout and a specific GC probability.
 * @param int $timeout The number of seconds until it should time out.
 * @param int $probability The probablity, in int percentage, that the garbage 
 *        collection routine will be triggered right now.
 * @param strint $cookie_domain The domain path for the cookie.
 */
function session_start_timeout($timeout=5, $probability=100, $cookie_domain='/') {
    // Set the max lifetime
    ini_set("session.gc_maxlifetime", $timeout);

    // Set the session cookie to timout
    ini_set("session.cookie_lifetime", $timeout);

    // Change the save path. Sessions stored in teh same path
    // all share the same lifetime; the lowest lifetime will be
    // used for all. Therefore, for this to work, the session
    // must be stored in a directory where only sessions sharing
    // it's lifetime are. Best to just dynamically create on.
    $seperator = strstr(strtoupper(substr(PHP_OS, 0, 3)), "WIN") ? "\\" : "/";
    $path = ini_get("session.save_path") . $seperator . "session_" . $timeout . "sec";
    if(!file_exists($path)) {
        if(!mkdir($path, 600)) {
            trigger_error("Failed to create session save path directory '$path'. Check permissions.", E_USER_ERROR);
        }
    }
    ini_set("session.save_path", $path);

    // Set the chance to trigger the garbage collection.
    ini_set("session.gc_probability", $probability);
    ini_set("session.gc_divisor", 100); // Should always be 100

    // Start the session!
    session_start();

    // Renew the time left until this session times out.
    // If you skip this, the session will time out based
    // on the time when it was created, rather than when
    // it was last used.
    if(isset($_COOKIE[session_name()])) {
        setcookie(session_name(), $_COOKIE[session_name()], time() + $timeout, $cookie_domain);
    }
}