WordPress Core Architecture Interview Questions

Answer: A high-level overview of what happens when a request hits WordPress:

1. index.php: Entry point.
2. wp-blog-header.php: Loads the environment.
3. wp-load.php: Bootstraps WordPress (config, db).
4. wp-settings.php: Loads core libraries, plugins, and the active theme's `functions.php`.
5. init hook: Core is fully loaded.
6. wp() (Main Query): Parses the URL and runs the main database query to determine what content to show.
7. template_redirect: Decides which template file to load.
8. template-loader.php: Loads the template (e.g., `single.php`).
9. Shutdown: PHP execution ends.

Answer: Both are "Hooks", but they serve different purposes:

• Actions (`do_action` / `add_action`): Events where you can "do" something at a specific point (e.g., echo HTML, send an email, update DB). They do not return a value.
• Filters (`apply_filters` / `add_filter`): Events where you modify data and must return it. (e.g., changing the post title, modifying a query).

// Action: Do something
add_action('wp_footer', function() {
    echo '<!-- Analytics Code -->';
});

// Filter: Modify data
add_filter('the_title', function($title) {
    return 'Prefix: ' . $title;
});

Answer: WordPress uses a relational database (MySQL/MariaDB/PostgreSQL) with 12 core tables (by default). The most important ones are:

• wp_posts: Stores posts, pages, attachments, nav menu items, and revisions.
• wp_postmeta: Stores metadata for posts (EAV model).
• wp_users & wp_usermeta: User data.
• wp_options: Global site settings.
• wp_terms, wp_term_taxonomy, wp_term_relationships: Categories and tags.

Key Concept: The EAV (Entity-Attribute-Value) model in `wp_postmeta` allows for flexibility but can be a performance bottleneck if not queried correctly.

Answer: It is the logic WordPress uses to decide which theme file to use to render a page. It searches from specific to generic.

Example for a Single Post:

1. single-{post_type}-{slug}.php
2. single-{post_type}.php
3. single.php
4. singular.php
5. index.php (Fallback)

Answer: Transients are a way to store cached data in the database (wp_options) with an expiration time. They are similar to Options but are temporary.

Usage: Storing the result of an expensive API call or complex query.

if ( false === ( $data = get_transient( 'my_api_data' ) ) ) {
    $data = fetch_api_data();
    set_transient( 'my_api_data', $data, 12 * HOUR_IN_SECONDS );
}

Answer:

• `WP_Query`: The class behind all queries. It is the most powerful and customizable way to query the database. It is safe to use anywhere.
• `get_posts()`: A wrapper around `WP_Query` that returns an array of post objects (not a loop). It suppresses filters by default and is slightly faster for simple retrievals.
• `query_posts()`: NEVER USE THIS. It overrides the main query loop and causes significant issues with pagination and page state. It is deprecated in practice.

Answer: This is an architecture question.

• Custom Post Type (CPT): Use this for Entities or "Nouns" that have their own content and lifecycle. Examples: Movies, Products, Events, Team Members.
• Custom Taxonomy: Use this for Grouping or "Adjectives" that describe those entities. Examples: Genre (for Movies), Color (for Products), Department (for Team Members).

Rule of Thumb: If you need to write a paragraph about it, it's a Post Type. If it's just a label to sort by, it's a Taxonomy.

Answer: The Singleton pattern ensures a class has only one instance and provides a global point of access to it.

WordPress Examples:

• $wpdb: The global database connection object.
• $wp_rewrite: Manages rewrite rules.
• Many plugins use a `getInstance()` method to prevent multiple initializations of their main class, which avoids hook duplication.

Answer: The Rewrite API allows you to create custom URL structures (permalinks) that map to specific WordPress queries.

Key Functions:

• add_rewrite_rule(): Adds a new rule to the top of the rewrite list.
• add_rewrite_tag(): Registers a query variable so WordPress knows to look for it.
• flush_rewrite_rules(): Regenerates the `.htaccess` file or database rules. Warning: Only run this on plugin activation/deactivation, never on every page load (performance killer).

Answer: WP-Cron is a pseudo-cron system. It is not a real system cron job. It runs when a visitor loads a page.

Limitation: If no one visits the site, scheduled tasks (like publishing scheduled posts) won't run.

Best Practice: Disable the default WP-Cron (`DISABLE_WP_CRON`) and set up a real system cron job to call `wp-cron.php` every minute.

Answer: The REST API provides a JSON interface to WordPress content. It allows you to build decoupled applications (Headless WP).

Extending: Use `register_rest_route()` to create custom endpoints.

add_action( 'rest_api_init', function () {
    register_rest_route( 'myplugin/v1', '/data', array(
        'methods' => 'GET',
        'callback' => 'my_custom_callback',
        'permission_callback' => function () { return current_user_can( 'edit_posts' ); }
    ) );
} );

Answer: "Number used ONCE". They are security tokens used to protect URLs and forms from misuse (CSRF attacks).

Usage:

• Create: `wp_create_nonce('action_name')`
• Verify: `wp_verify_nonce($_REQUEST['_wpnonce'], 'action_name')`

Note: In WordPress, nonces are valid for 24 hours by default, so they are not strictly "once", but they are user-specific.

Answer: `WP_Query` is the class behind all post queries. The main loop uses a global instance of it.

Key Methods:

• `have_posts()`: Checks if there are posts to loop through.
• `the_post()`: Sets up the global `$post` object and advances the counter.
• `wp_reset_postdata()`: Restores the global `$post` to the main query after a secondary loop.