How to Understand the Return Value of SessionHandler::create_sid()? What Debugging Techniques Can Help Developers?

In PHP, when customizing the session handling mechanism, we can implement the SessionHandlerInterface or extend the SessionHandler class to define custom session storage logic. Among these, SessionHandler::create_sid() is an overridable method used to generate the session ID. Understanding the return value of this method and its usage is especially important for debugging complex session mechanisms.

1. What Does SessionHandler::create_sid() Do?

create_sid() is automatically called when session_start() is invoked and there is no valid session ID present. Its purpose is to return a new, unique session ID string. The default implementation generates a high-entropy ID based on session.sid_length and session.sid_bits_per_character, but you can also customize the logic to control how the session ID is generated.

class MySessionHandler extends SessionHandler {
    public function create_sid(): string {
        return hash('sha256', random_bytes(32));
    }
}
<p>session_set_save_handler(new MySessionHandler(), true);<br>
session_start();<br>

In the above code, we use SHA-256 to hash 32 bytes of random data to generate a unique session ID. The returned value looks like this:

82c4ad45fef0c9f0ed72cd3e78c0f5e5c7e35a8f70e94dfd6a5f1a15f2b19e73

2. What Requirements Should the Return Value Meet?

The string returned by create_sid() must meet the following criteria:

1. Uniqueness: It must not repeat within a certain time frame.

2. Unpredictability: It should be hard to guess, preventing session hijacking.

3. Compatibility with session.use_strict_mode=1: When strict mode is enabled, if the returned session ID already exists in storage, it will be rejected and regenerated.

Failure to meet these conditions may lead to session conflicts or security vulnerabilities.

3. Debugging Techniques for create_sid() Return Values

The most direct way to debug create_sid() is to temporarily add logging and tracing information:

1. Log the Generated Session ID

public function create_sid(): string {
    $sid = hash('sha256', random_bytes(32));
    error_log("New session ID created: $sid");
    return $sid;
}

This will write each generated SID to the PHP error log, usually located at /var/log/php_errors.log or configured via php.ini.

2. Use Browser Developer Tools to Check the Set-Cookie Header

In the browser, use developer tools (such as Chrome’s Network tab) to view the Set-Cookie response header and confirm whether the server returns the expected session ID.

For example, the response header might look like this:

Set-Cookie: PHPSESSID=82c4ad45fef0c9f0ed72cd3e78c0f5e5c7e35a8f70e94dfd6a5f1a15f2b19e73; path=/; HttpOnly

3. Verify Session Creation in Storage

Check the backend storage (such as Redis, filesystem, or database) to see if the session data with this ID is actually saved. For example, if you save sessions to Redis:

$sessionKey = "PHPREDIS_SESSION:sess_$sid";

You can use the command:

GET PHPREDIS_SESSION:sess_82c4ad45fef0c9f0ed72cd3e78c0f5e5c7e35a8f70e94dfd6a5f1a15f2b19e73

to check if the session key exists.

4. Create a Debug Page for Output

You can temporarily create a debug page to view the current session ID and its status:

session_start();
echo "<pre>Current Session ID: " . session_id() . "

Session Content: ";
print_r($_SESSION);
echo "

After visiting the page, the output might look like this:

Current Session ID: 82c4ad45fef0c9f0ed72cd3e78c0f5e5c7e35a8f70e94dfd6a5f1a15f2b19e73
Session Content: Array
(
)

5. Use URL-Based Session ID (For Debugging Only)

If the client does not support cookies, you can append the session ID to the URL for debugging purposes:

https://gitbox.net/debug.php?PHPSESSID=82c4ad45fef0c9f0ed72cd3e78c0f5e5c7e35a8f70e94dfd6a5f1a15f2b19e73

Note: This method is only suitable for development debugging. In production environments, passing session IDs via URL should be disabled to prevent session leakage.

4. Additional Suggestions for Debugging

• Enable session.use_strict_mode=1 to force PHP to reject existing session IDs.

• Configure session.save_path to a directory with easy read/write access to check raw session files.

• Temporarily enable higher error reporting and logging levels in php.ini or .htaccess.

error_reporting = E_ALL
display_errors = On
log_errors = On
error_log = /tmp/php_error.log