Session caching issue

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### 🔴 Why `session_start()` Breaks Caching
When you start a PHP session:

– It sends a `Set-Cookie` header with `PHPSESSID`.
– Pages become user-specific.
– SiteGround’s cache system (Dynamic Cache) **automatically skips** caching when it detects a session.

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### ✅ Recommended Alternatives

#### Option 1: **Use cookies instead of PHP sessions**
If you just need to store small user-specific values (like current step in a form, language, etc.), prefer `setcookie()` and `$_COOKIE`.

// Set cookie
setcookie('my_key', 'my_value', time() + 3600, '/');

// Read cookie
$value = $_COOKIE['my_key'] ?? '';

Cookies **do not break caching**.

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#### Option 2: **Use session only on specific pages**
If you must use `$_SESSION`, restrict it only where necessary, like on a specific page:

add_action('template_redirect', 'csos_conditional_session');
function csos_conditional_session() {
    if (is_page(['form-step', 'checkout'])) {
        if (!session_id()) {
            session_start();
        }
    }
}

This way, 95% of your site remains cacheable, and only specific dynamic pages avoid cache.

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#### Option 3: **AJAX with transient/cache or database**
If you need to store temporary data between steps or user actions, consider:

– `update_user_meta()` (if user is logged in)
– `update_option()` (site-wide, not per user)
– `set_transient()` for time-limited data
– Storing step/session-like data in the frontend (JS/localStorage), and saving only on final submit

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### 🔐 Summary Table

| Method | Caching Safe? | Suitable For |
|—————-|—————-|——————————-|
| `$_SESSION` | ❌ No | Logged-in admin/backend use |
| `$_COOKIE` | ✅ Yes | Frontend user state |
| Transients | ✅ Yes | Temporary cache (non-user) |
| `localStorage` | ✅ Yes | Multi-step forms (frontend JS)|
| Conditional `session_start()` | 🟡 Partially | Limit to specific pages |

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### ✅ Recommendation for SiteGround
Update your session logic to:

add_action('template_redirect', 'csos_conditional_session');
function csos_conditional_session() {
    if (is_page('form-page') || is_page('checkout')) {
        if (!session_id()) {
            session_start();
        }
    }
}

Or switch to cookies for non-sensitive data.

Let me know your exact use case and I’ll suggest the best caching-safe alternative.

That means if you're relying on `$_SESSION` **within AJAX handlers**, the session won’t be available unless it's started **during the AJAX call itself**.

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### ✅ Updated Approach: Handle both page and AJAX requests

Use both `template_redirect` **and** a hook that runs during AJAX requests like `admin_init` or `init` — but only conditionally for your custom use case:

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### 🔄 Solution

add_action('init', 'csos_maybe_start_session');

function csos_maybe_start_session() {
    // Start session only if not already started
    if (session_id()) {
        return;
    }

    // 1. AJAX call
    if (defined('DOING_AJAX') && DOING_AJAX) {
        session_start();
        return;
    }

    // 2. Frontend specific pages
    if (!is_admin() && (is_page('form-page') || is_page('checkout'))) {
        session_start();
    }
}

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### ✅ Why this works:

– ✅ **Avoids starting session on all pages** (preserves caching)
– ✅ **Supports AJAX** by checking `DOING_AJAX`
– ✅ **Starts session only when really needed**

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### 🔍 Tip for Debugging:

To ensure session works in your AJAX handler, add this at the top of your callback temporarily:

error_log('Session ID: ' . session_id());
error_log(print_r($_SESSION, true));

Or return `$_SESSION` in the AJAX response to debug.

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Let me know if you're using a custom AJAX handler (`wp_ajax_` or `wp_ajax_nopriv_`) — I can tailor the code further if needed.

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### ✅ Final Solution: Handle `wp_ajax_` and specific frontend pages without breaking caching

Add this in your plugin:

add_action('init', 'csos_maybe_start_session');
function csos_maybe_start_session() {
    // If session already started, do nothing
    if (session_id()) {
        return;
    }

    // ✅ 1. For AJAX calls (both logged in and not logged in)
    if (defined('DOING_AJAX') && DOING_AJAX) {
        session_start();
        return;
    }

    // ✅ 2. For specific frontend pages only
    if (!is_admin() && (is_page('form-page') || is_page('checkout'))) {
        session_start();
    }
}

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### ✅ Additional Notes

– ✅ Works with `wp_ajax_` and `wp_ajax_nopriv_`
– ✅ Keeps caching intact on all other pages
– ✅ Avoids session on admin screens unless needed (safe for wp-admin performance)

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### 🧪 Example AJAX usage with session

If you're using something like:

add_action('wp_ajax_save_step_data', 'save_step_data');
add_action('wp_ajax_nopriv_save_step_data', 'save_step_data');

function save_step_data() {
    session_start(); // just to be safe, even though already handled
    $_SESSION['step_data'] = $_POST['data'] ?? '';

    wp_send_json_success([
        'stored' => $_SESSION['step_data']
    ]);
}

Make sure your JavaScript calls this properly via `admin-ajax.php`.

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Let me know your exact AJAX action name if you want me to write a full working snippet.