Hooks, Actions & Filters

// Example: Add custom JavaScript to the front-end
add_action( 'wp_enqueue_scripts', 'my_custom_scripts' );
function my_custom_scripts() {
    wp_enqueue_script( 
        'my-script', 
        get_stylesheet_directory_uri() . '/js/custom.js',
        array( 'jquery' ),
        '1.0',
        true  // Load in footer
    );
}

// Example: Run code when WordPress initializes
add_action( 'init', 'my_init_function' );
function my_init_function() {
    // Register custom post types here
    // This runs after WordPress loads but before headers are sent
}

// Example: Add content after every post
add_filter( 'the_content', 'add_cta_after_content' );
function add_cta_after_content( $content ) {
    if ( is_single() && is_main_query() ) {
        $cta = '<div class="post-cta">Thanks for reading! Subscribe for more.</div>';
        $content .= $cta;
    }
    return $content;  // MUST return the modified content
}

// Example: Change excerpt length
add_filter( 'excerpt_length', 'custom_excerpt_length' );
function custom_excerpt_length( $length ) {
    return 30;  // Return 30 words instead of default 55
}

// Example: Modify login page logo URL
add_filter( 'login_headerurl', 'custom_login_url' );
function custom_login_url() {
    return home_url();  // Link to homepage instead of wordpress.org
}

// Example: Priority in action
add_action( 'wp_head', 'my_early_function', 5 );   // Runs early
add_action( 'wp_head', 'my_late_function', 20 );   // Runs late

// Example: Multiple arguments in filter
add_filter( 'the_title', 'modify_title', 10, 2 );
function modify_title( $title, $post_id ) {
    // $title is first argument, $post_id is second
    if ( $post_id === 42 ) {
        $title = 'Special: ' . $title;
    }
    return $title;
}