Actions and Filters

Actions vs Filters

# ACTIONS -- side effects, no return value needed
function send_welcome_email($user_id) {
    $user = get_userdata($user_id);
    wp_mail($user->user_email, 'Welcome!', 'Thanks for joining.');
}
add_action('user_register', 'send_welcome_email');

# FILTERS -- transform a value, MUST return it
function add_copyright($content) {
    if (is_single()) {
        $content .= '
© ' . date('Y') . ' My Company
';
    }
    return $content; // MUST return!
}
add_filter('the_content', 'add_copyright');

# Priority (lower = runs first, default = 10)
add_action('init', 'my_function', 5);

# Remove hooks
remove_action('wp_head', 'wp_generator');
remove_filter('the_content', 'wpautop');

# Key difference:
# add_action -- do something at a point
# add_filter -- modify something and return it