Displaying Errors from the save_post Hook in WordPress

Handling and Displaying Errors from the WordPress save_post Hook: Three Approaches

WordPress lacks a single, standardized method for managing and displaying errors originating from the save_post hook. However, several effective strategies exist, each with its own strengths and weaknesses. This article explores three prominent approaches: using the $_SESSION global, leveraging WordPress transients, and employing GET parameters in redirects.

The complexity stems from WordPress's post-saving redirect. After the save_post hook executes, the user is redirected, breaking the execution thread and losing access to global variables. Therefore, a mechanism is needed to transfer error information from the save action to the redirected page.

Method 1: Utilizing the $_SESSION Global

This straightforward method stores the error message in the $_SESSION global variable. It's simple to implement and avoids database interaction.

• Implementation:

if ( !session_id() ) {
    session_start();
}

if ($error) {
    $_SESSION['my_plugin_errors'] = $error->get_error_message();
}

// ...in admin_notices hook...
add_action( 'admin_notices', 'my_error_message' );
function my_error_message() {
    if ( isset( $_SESSION['my_plugin_errors'] ) ) {
        ?>
        <div class="error">
            <p><?php echo $_SESSION['my_plugin_errors']; ?></p>
        </div>
        <?php
        unset( $_SESSION['my_plugin_errors'] );
    }
}

• Pros: Easy implementation, no database overhead.
• Cons: Not a standard WordPress practice; relies on session handling, which might not be consistently enabled across all setups.

Method 2: Employing WordPress Transients

Transients provide a WordPress-native caching mechanism. They store data with an expiration time, utilizing the object cache if available, or falling back to the database.

• Implementation:

if ($error) {
    set_transient("my_save_post_errors_{$post_id}_{$user_id}", $error, 45);
}

// ...in admin_notices hook...
add_action( 'admin_notices', 'my_error_message' );
function my_error_message() {
    if ( $error = get_transient( "my_save_post_errors_{$post_id}_{$user_id}" ) ) {
        ?>
        <div class="error">
            <p><?php echo $error->get_error_message(); ?></p>
        </div>
        <?php
        delete_transient("my_save_post_errors_{$post_id}_{$user_id}");
    }
}

• Pros: WordPress-friendly, automatic cleanup of expired data.
• Cons: Database interaction if an object cache isn't configured; potential data loss in edge cases.

Method 3: Appending a GET Parameter to the Redirect URL

This mirrors WordPress's own approach for displaying update messages. The error code is added as a query parameter to the redirect URL.

• Implementation:

if ($error) {
    add_filter('redirect_post_location', function( $location ) use ( $error ) {
        return add_query_arg( 'my-plugin-error', $error->get_error_code(), $location );
    });
}

// ...in admin_notices hook...
add_action( 'admin_notices', 'my_error_message' );
function my_error_message() {
    if ( isset( $_GET['my-plugin-error'] ) ) {
        // Handle error based on error code (switch statement)
        ?>
        <div class="error">
            <p><?php  // Display error message based on $_GET['my-plugin-error'] ?></p>
        </div>
        <?php
    }
}

• Pros: High performance, no database access.
• Cons: Requires duplicating error messages; maintenance overhead can increase with a large number of error codes.

Conclusion

The optimal method depends on the specific context. Transients offer a good balance between WordPress integration and performance, while the GET parameter approach provides the best performance but requires more careful management of error messages. The $_SESSION method is simplest but less reliable in diverse WordPress environments. Choose the method that best suits your needs and project scale.

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