How to Check if PHP session has already started?

In PHP, we utilize session_start() to start a session. However, calling this function multiple times in the same script throws an error. Here we will learn how to check if a session has already started without calling session_start() twice.

There are two methods to resolve this problem, depending on your PHP version.

Method 1: Using session_id() (PHP < 5.4.0)

For PHP versions below 5.4.0, use the session_id() function to check if a session exists −

<?php
    if(session_id() == '') {
        session_start();
    }
    
    // Your session code here
    $_SESSION['username'] = 'john_doe';
    echo "Session started successfully!";
?>

The session_id() function returns an empty string if no session has been started, making it perfect for conditional session initialization.

Method 2: Using session_status() (PHP ≥ 5.4.0)

For PHP 5.4.0 and above, use the session_status() function which returns the current session status −

<?php
    if (session_status() == PHP_SESSION_NONE) {
        session_start();
    }
    
    // Your session code here
    $_SESSION['user_id'] = 123;
    echo "Session is active!";
?>

Session Status Constants

The session_status() function returns one of these predefined constants −

Complete Example

Here's a practical example showing both methods in a reusable function −

<?php
function safe_session_start() {
    if (version_compare(phpversion(), '5.4.0', '>=')) {
        if (session_status() == PHP_SESSION_NONE) {
            session_start();
        }
    } else {
        if(session_id() == '') {
            session_start();
        }
    }
}

// Usage
safe_session_start();
$_SESSION['message'] = 'Session started safely!';
echo $_SESSION['message'];
?>

Conclusion

Use session_status() for modern PHP versions (5.4.0+) and session_id() for older versions. Both methods prevent the "session already started" error effectively.