Are transients private or public?

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Transients are just database keys that expire. It’s like telling WordPress you want it to remember a certain piece of information, but for a limited time. In general, transients are accessible to PHP through any request.

But since they’re server-side, transients are only exposed to front-end users if you as the developer expose them.

A Solution

Given your specific question:

I load the home page which generates a random number and stores it into a transient which should be accessible within that same session.

I would recommend pairing Transients with cookies. Basically, generate a unique key for every request (perhaps a hash of time() and some randome number) and use that as the user’s session key. Then, you can store a transient for just that user. Be sure to pass this unique key to the browser in the way of a cookie, and the next time that same user refreshes the page, they’ll get the same number.

To give you some untested pseudo-code …

/**
 * Check the user's cookie if they have it.
 * Create one if they don't.
 */
function wpa_105249_check_cookie() {
    if ( isset( $_COOKIE['wpa_105249_key'] ) ) {
        $key = (string) $_COOKIE['wpa_105249_key'];
    } else {
        $key = md5( time() . rand() );

        // Set the browser cookie to expire in 30 minutes
        set_cookie( 'wpa_105249_key', $key, time() + 30 * 60 )
    }

    // Try to grab the transient from the database, if it exists.
    $transient = get_transient( 'wpa_105249_' . $key );

    // If the transient doesn't exist, create it.
    if ( false === $transient ) {
        // Call your function to generate the random number.
        $transient = generate_random_number();

        // Store the transient, but expire in 30 minutes.
        set_transient( 'wpa_105249_' . $key, $transient, 30 * 60 );
    }

    wp_cache_set( 'wpa_105249_number', $transient );
}
add_action( 'plugins_loaded', 'wpa_105249_check_cookie' );

In plain English, this will:

  1. Check for the wpa_105249_key cookie in the request
    • If the cookie doesn’t exist, create one.
    • Set the cookie in the page’s response headers so the browser has it for next time.
  2. Attempt to grab a transient as identified by the cookie
    • If the transient doesn’t exist, generate a new random number using your function and store it in the transient for next time.
  3. Set the random number in the request cache

Any code you call after this – basically anywhere else in your code – you can grab the current user’s random number simply by calling:

$number = wp_cache_get( 'wpa_105249_number' );

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