When I started writing WordPress code, hooks were my biggest "aha" moment. Almost every customisation — adding scripts, changing content, extending forms — goes through hooks, so you never need to edit core.

What are hooks?

WordPress runs through a series of "stops". At each one it asks: does anyone want to do something here? You register a function at that stop. Two kinds:

• Actions: do something extra at that moment (print HTML, send email, enqueue scripts).
• Filters: receive a value, change it, return it (titles, excerpt length, content).

Actions

add_action( 'wp_enqueue_scripts', function () {
    wp_enqueue_style( 'hp-main', get_stylesheet_directory_uri() . '/assets/main.css', [], '1.0' );
} );
 
add_action( 'wp_footer', function () {
    echo '<!-- Made by HoangPham -->';
} );

Filters

Filters must return a value — forgetting return was my most common early mistake, and the content simply vanished.

add_filter( 'excerpt_length', function ( $length ) {
    return 30;
} );
 
add_filter( 'the_content', function ( $content ) {
    if ( is_single() ) {
        $content .= '<p class="note">Thanks for reading!</p>';
    }
    return $content;
} );

Priority and arguments

add_filter( 'the_title', 'hp_title', 20, 2 );

• Third argument (priority, default 10): lower runs first.
• Fourth: how many arguments your function accepts (default 1).

Removing a hook

Use remove_action/remove_filter with the exact function name and priority, after the hook has been added (usually on init or after_setup_theme).

Where to find hooks

• The Code Reference at developer.wordpress.org.
• Search a theme/plugin for do_action( and apply_filters(.
• Debug plugins like Query Monitor show hooks fired on a page.

Mistakes I made

1. A filter without return → empty content.
2. Removing a hook with the wrong priority → hours of debugging.
3. Heavy logic on hooks that run everywhere (init) → a slow site.