add_rewrite_rule( string $regex, string|array $query, string $after = 'bottom' )

// Maps /nutrition/apples/ to index.php?nutrition=apples
add_action( 'init', function() {
    add_rewrite_rule(
        '^nutrition/([^/]+)/?$',
        'index.php?nutrition=$matches[1]',
        'top'
    );
});

// Register the query var
add_filter( 'query_vars', function( $vars ) {
    $vars[] = 'nutrition';
    return $vars;
});

add_rewrite_rule(
    '^products/([^/]+)/reviews/?$',
    array(
        'post_type' => 'product',
        'name'      => '$matches[1]',
        'reviews'   => '1',
    ),
    'top'
);

// /events/2024/01/ → Events from January 2024
add_rewrite_rule(
    '^events/([0-9]{4})/([0-9]{2})/?$',
    'index.php?post_type=event&year=$matches[1]&monthnum=$matches[2]',
    'top'
);

add_rewrite_tag( string $tag, string $regex, string $query = '' )

add_action( 'init', function() {
    // Creates %product% tag matching any non-slash string
    add_rewrite_tag( '%product%', '([^/]+)', 'product=' );
});

add_action( 'init', function() {
    // Register CPT with custom permalink structure
    register_post_type( 'book', array(
        'public'   => true,
        'rewrite'  => array( 'slug' => 'library/%author_name%' ),
    ));
    
    // Add custom tag for author name in URL
    add_rewrite_tag( '%author_name%', '([^/]+)', 'author_name=' );
});

// Filter the permalink to replace the tag
add_filter( 'post_type_link', function( $link, $post ) {
    if ( 'book' === $post->post_type ) {
        $author = get_post_meta( $post->ID, 'book_author', true );
        $link = str_replace( '%author_name%', sanitize_title( $author ), $link );
    }
    return $link;
}, 10, 2 );

// When $query is empty, 'city' becomes a registered query var
add_rewrite_tag( '%city%', '([^/]+)' );

// Now you can use get_query_var( 'city' )

remove_rewrite_tag( string $tag )

add_action( 'init', function() {
    remove_rewrite_tag( '%author%' );
}, 20 ); // Later priority to ensure tag exists

add_permastruct( string $name, string $struct, array $args = array() )

add_action( 'init', function() {
    add_permastruct( 'portfolio', 'work/%portfolio%', array(
        'with_front' => false,
        'ep_mask'    => EP_PERMALINK,
        'paged'      => true,
        'feed'       => true,
    ));
});

add_action( 'init', function() {
    add_permastruct( 'genre', 'music/genre/%genre%', array(
        'with_front' => false,
        'ep_mask'    => EP_CATEGORIES,
    ));
});

add_permastruct( 'simple', 'simple/%name%', array(
    'paged' => false,
    'feed'  => false,
));

remove_permastruct( string $name )

add_action( 'init', function() {
    remove_permastruct( 'portfolio' );
}, 20 );

add_feed( string $feedname, callable $callback ): string

add_action( 'init', function() {
    add_feed( 'json', 'render_json_feed' );
});

function render_json_feed( $is_comment_feed, $feed_type ) {
    header( 'Content-Type: application/json; charset=' . get_option( 'blog_charset' ) );
    
    $posts = get_posts( array( 'posts_per_page' => 10 ) );
    $items = array();
    
    foreach ( $posts as $post ) {
        $items[] = array(
            'title'   => $post->post_title,
            'url'     => get_permalink( $post ),
            'content' => $post->post_content,
        );
    }
    
    echo wp_json_encode( array( 'items' => $items ) );
}

add_feed( 'myfeed', function( $is_comment_feed, $feed_type ) {
    load_template( get_template_directory() . '/feed-myfeed.php' );
});

add_rewrite_endpoint( string $name, int $places, string|bool $query_var = true )

add_action( 'init', function() {
    add_rewrite_endpoint( 'api', EP_PERMALINK );
});

// Handle the endpoint
add_action( 'template_redirect', function() {
    $api = get_query_var( 'api' );
    if ( $api !== '' ) {
        // Output JSON instead of template
        wp_send_json( array(
            'post_id' => get_the_ID(),
            'title'   => get_the_title(),
        ));
    }
});

add_action( 'init', function() {
    add_rewrite_endpoint( 'print', EP_PERMALINK | EP_PAGES );
});

add_filter( 'template_include', function( $template ) {
    if ( get_query_var( 'print' ) !== false && get_query_var( 'print' ) !== '' ) {
        return get_template_directory() . '/print-template.php';
    }
    return $template;
});

// Don't register query var (handle differently)
add_rewrite_endpoint( 'custom', EP_ROOT, false );

// Creates URLs like /post-name/format/json/
add_rewrite_endpoint( 'format', EP_PERMALINK );

// Access the value
$format = get_query_var( 'format' ); // 'json' from /post/format/json/

flush_rewrite_rules( bool $hard = true )

register_activation_hook( __FILE__, 'myplugin_activate' );
function myplugin_activate() {
    // Register post types/taxonomies first
    myplugin_register_post_types();
    
    // Then flush
    flush_rewrite_rules();
}

register_deactivation_hook( __FILE__, 'myplugin_deactivate' );
function myplugin_deactivate() {
    flush_rewrite_rules();
}

flush_rewrite_rules( false );

// BAD - causes performance issues
add_action( 'init', function() {
    add_rewrite_rule( '...', '...' );
    flush_rewrite_rules(); // DON'T DO THIS
});

// GOOD - flush only on activation
register_activation_hook( __FILE__, function() {
    myplugin_add_rewrite_rules();
    flush_rewrite_rules();
});

url_to_postid( string $url ): int

$post_id = url_to_postid( 'https://example.com/hello-world/' );
if ( $post_id ) {
    $post = get_post( $post_id );
}

$referer = wp_get_referer();
if ( $referer ) {
    $post_id = url_to_postid( $referer );
}

function is_internal_post_link( $url ) {
    return url_to_postid( $url ) > 0;
}

wp_resolve_numeric_slug_conflicts( array $query_vars = array() ): array

_wp_filter_taxonomy_base( string $base ): string