CUSTOM FILTER WIDGET: ProductFilter.php

REGISTERATION (gutenberg block)
register_block_type('custom/product-filter', array(
  'render_callback' => 'render_product_filter',
  'attributes' => array(
    'taxonomy' => 'pa_color',
    'maxResults' => 500
  )
));

FILTER LOGIC
• Query: WC_Product_Query with tax_query for pa_color and pa_size
• Sanitize inputs: sanitize_text_field() on $_GET['color'], validate against registered attributes
• Escape output: wp_kses_allowed_html() on frontend, never trust user input

REST API ENDPOINT (AJAX alternative)
GET /wp-json/custom/v1/products?color=red&size=large
• Nonce verification on every call
• Rate limit: 30 requests/minute per IP (prevent scraping)
• Cache: 1 hour Redis TTL for popular filter combinations

SECURITY HARDENING
• No SELECT * queries; fetch only ID, name, price, image URL
• Prepared statements for all database queries
• Check user capabilities before exposing private product data
• Escape all echoed values wp_kses_post() for attribute output

PERFORMANCE TUNING
• Index: wp_postmeta(meta_key='pa_color') for fast lookups
• Lazy load color swatches (first 12 options, paginate via AJAX)
• Compress SVG color swatches to data URIs (no extra HTTP requests)

CROSS-SELL INTEGRATION
• If user filters to 1 product: show "similar items in red size M" upsell below