What are the best practices for WordPress custom theme development?

WordPress custom theme development requires following WordPress coding standards and best practices. First, create a theme folder containing required files: style.css (theme metadata), index.php (main template), functions.php (theme functions), header.php, footer.php, sidebar.php. Add theme information comments in style.css including theme name, version, author, description, etc. Use wp_enqueue_style() and wp_enqueue_scripts() hooks to properly load CSS and JS files, avoiding direct inclusion in template files. Use wp_head() and wp_footer() functions to ensure WordPress core functionality works properly. Implement template hierarchy by creating single.php (single post), page.php (page), archive.php (archive), category.php (category), tag.php (tag), search.php (search results), 404.php (error page) and other template files. Use WordPress template tags like the_post(), the_title(), the_content(), the_excerpt() to output content. Use wp_nav_menu() to register and display custom menus. Use register_sidebar() and dynamic_sidebar() to create and display widget areas. Use add_theme_support() to enable theme features such as post-thumbnails (featured images), html5 (HTML5 support), title-tag (title tag), custom-logo (custom logo), etc. Use get_template_part() to include template fragments for better code reuse. Use escaping functions like esc_html(), esc_attr(), esc_url() to prevent XSS attacks. Use wp_kses_post() to filter allowed HTML tags. Use helper functions like checked(), selected(), disabled() to output form attributes. Use conditional tags like is_home(), is_front_page(), is_single(), is_page(), is_category() to determine current page type. Use get_header(), get_footer(), get_sidebar() to include template files. Use wp_link_pages() to output post pagination links. Use the_posts_pagination() or the_posts_navigation() to output pagination navigation. Use comments_template() to load comment template. Use wp_list_comments() to output comment list. Use comment_form() to output comment form. Use get_template_directory_uri() and get_stylesheet_directory_uri() to get theme directory URLs. Use get_template_directory() and get_stylesheet_directory() to get theme directory paths. Use locate_template() to find template files. Use apply_filters() and do_action() to create extensible themes. Use add_image_size() to create custom image sizes. Use add_editor_style() to add custom styles for the editor. Use load_theme_textdomain() to implement theme internationalization. Use __('string', 'textdomain') and _e('string', 'textdomain') to output translatable strings. Use wp_localize_script() to pass PHP data to JavaScript. Use add_action('after_setup_theme', 'theme_setup') hook to initialize theme settings. Use add_action('wp_enqueue_scripts', 'theme_scripts') hook to load scripts and styles. Use add_action('widgets_init', 'theme_widgets') hook to register widget areas. Use add_action('init', 'theme_custom_post_types') hook to register custom post types. Use add_action('init', 'theme_custom_taxonomies') hook to register custom taxonomies. Use add_shortcode('shortcode_name', 'shortcode_callback') to create shortcodes. Use add_filter('excerpt_length', 'custom_excerpt_length') to modify excerpt length. Use add_filter('excerpt_more', 'custom_excerpt_more') to modify excerpt ending text. Use add_filter('the_content', 'custom_content_filter') to filter post content. Use add_filter('body_class', 'custom_body_class') to add custom body classes. Use add_filter('post_class', 'custom_post_class') to add custom post classes. Use add_action('wp_head', 'custom_head_meta') to add custom head metadata. Use add_action('wp_footer', 'custom_footer_scripts') to add custom footer scripts. Use add_action('admin_init', 'theme_admin_settings') to create theme settings page. Use add_theme_page() to add theme options menu. Use register_setting() to register theme settings. Use add_settings_section() to add settings sections. Use add_settings_field() to add settings fields. Use get_option() and update_option() to read and update theme options. Use wp_customize API to create custom theme customizer options. Use WP_Customize_Control, WP_Customize_Color_Control, WP_Customize_Image_Control, etc. to create custom controls. Use add_action('customize_register', 'theme_customize_register') hook to register customizer options. Use get_theme_mod() and set_theme_mod() to read and update theme customizer options. Use add_action('wp_ajax_nopriv_ajax_action', 'ajax_callback') and add_action('wp_ajax_ajax_action', 'ajax_callback') to create AJAX handler functions. Use wp_send_json_success() and wp_send_json_error() to return JSON responses. Use check_ajax_referer() to verify AJAX requests. Use wp_die() to terminate AJAX requests. Use add_action('rest_api_init', 'theme_register_rest_routes') hook to register REST API routes. Use register_rest_route() to create custom REST endpoints. Use WP_REST_Request and WP_REST_Response to handle REST requests and responses. Use permission_callback parameter to verify REST request permissions. Use add_action('template_redirect', 'custom_template_redirect') hook to redirect templates. Use template_include filter to modify template file paths. Use add_filter('template_include', 'custom_template_include') hook to include custom templates. Use get_page_template() to get page template. Use get_single_template() to get single post template. Use get_archive_template() to get archive template. Use get_category_template() to get category template. Use get_tag_template() to get tag template. Use get_search_template() to get search template. Use get_404_template() to get 404 template. Use get_attachment_template() to get attachment template. Use get_tax_template() to get taxonomy template. Use get_custom_post_template() to get custom post type template. Use wp_get_theme() to get theme object. Use get_theme_file_path() and get_theme_file_uri() to get theme file paths and URLs. Use get_parent_theme_file_path() and get_parent_theme_file_uri() to get parent theme file paths and URLs. Use is_child_theme() to check if it's a child theme. Use get_template() to get parent theme directory name. Use get_stylesheet() to get current theme directory name. Use add_action('switch_theme', 'theme_switch_callback') hook to execute actions when theme is switched. Use add_action('after_switch_theme', 'theme_after_switch_callback') hook to execute actions after theme is switched. Use add_action('wp_delete_site', 'theme_delete_site_callback') hook to execute actions when site is deleted. Use add_action('wp_initialize_site', 'theme_initialize_site_callback') hook to execute actions when site is initialized. Use add_action('wp_insert_site', 'theme_insert_site_callback') hook to execute actions when site is inserted. Use add_action('wp_update_site', 'theme_update_site_callback') hook to execute actions when site is updated. Use add_action('wp_delete_post', 'theme_delete_post_callback') hook to execute actions when post is deleted. Use add_action('wp_insert_post', 'theme_insert_post_callback') hook to execute actions when post is inserted. Use add_action('wp_update_post', 'theme_update_post_callback') hook to execute actions when post is updated. Use add_action('save_post', 'theme_save_post_callback') hook to execute actions when post is saved. Use add_action('publish_post', 'theme_publish_post_callback') hook to execute actions when post is published. Use add_action('transition_post_status', 'theme_transition_post_status_callback') hook to execute actions when post status transitions. Use add_action('pre_get_posts', 'theme_pre_get_posts_callback') hook to modify query parameters before querying posts. Use add_action('the_post', 'theme_the_post_callback') hook to execute actions after processing post. Use add_filter('posts_where', 'theme_posts_where_callback') filter to modify WHERE clause. Use add_filter('posts_join', 'theme_posts_join_callback') filter to modify JOIN clause. Use add_filter('posts_orderby', 'theme_posts_orderby_callback') filter to modify ORDER BY clause. Use add_filter('posts_groupby', 'theme_posts_groupby_callback') filter to modify GROUP BY clause. Use add_filter('posts_fields', 'theme_posts_fields_callback') filter to modify SELECT fields. Use add_filter('posts_limits', 'theme_posts_limits_callback') filter to modify LIMIT clause. Use add_filter('posts_distinct', 'theme_posts_distinct_callback') filter to add DISTINCT keyword. Use add_filter('post_limits_request', 'theme_post_limits_request_callback') filter to modify query limits. Use add_filter('found_posts_query', 'theme_found_posts_query_callback') filter to modify query results. Use add_filter('the_posts', 'theme_the_posts_callback') filter to modify post array. Use add_filter('post_class', 'theme_post_class_callback') filter to modify post classes. Use add_filter('post_thumbnail_html', 'theme_post_thumbnail_html_callback') filter to modify featured image HTML. Use add_filter('get_the_excerpt', 'theme_get_the_excerpt_callback') filter to modify excerpt content. Use add_filter('the_content', 'theme_the_content_callback') filter to modify post content. Use add_filter('the_title', 'theme_the_title_callback') filter to modify post title. Use add_filter('get_the_terms', 'theme_get_the_terms_callback') filter to modify taxonomy terms. Use add_filter('get_term_link', 'theme_get_term_link_callback') filter to modify taxonomy link. Use add_filter('term_link', 'theme_term_link_callback') filter to modify taxonomy link. Use add_filter('get_pagenum_link', 'theme_get_pagenum_link_callback') filter to modify pagination link. Use add_filter('next_post_link', 'theme_next_post_link_callback') filter to modify next post link. Use add_filter('previous_post_link', 'theme_previous_post_link_callback') filter to modify previous post link. Use add_filter('get_comments_number', 'theme_get_comments_number_callback') filter to modify comment count. Use add_filter('comments_open', 'theme_comments_open_callback') filter to modify comment open status. Use add_filter('pings_open', 'theme_pings_open_callback') filter to modify ping open status. Use add_filter('comments_template', 'theme_comments_template_callback') filter to modify comment template. Use add_filter('comment_form_defaults', 'theme_comment_form_defaults_callback') filter to modify comment form defaults. Use add_filter('comment_text', 'theme_comment_text_callback') filter to modify comment content. Use add_filter('get_comment_author', 'theme_get_comment_author_callback') filter to modify comment author. Use add_filter('get_comment_date', 'theme_get_comment_date_callback') filter to modify comment date. Use add_filter('get_comment_time', 'theme_get_comment_time_callback') filter to modify comment time. Use add_filter('comment_reply_link', 'theme_comment_reply_link_callback') filter to modify comment reply link. Use add_filter('cancel_comment_reply_link', 'theme_cancel_comment_reply_link_callback') filter to modify cancel reply link. Use add_filter('comment_form_submit_button', 'theme_comment_form_submit_button_callback') filter to modify comment submit button. Use add_filter('comment_form_submit_field', 'theme_comment_form_submit_field_callback') filter to modify comment submit field. Use add_filter('comment_form_fields', 'theme_comment_form_fields_callback') filter to modify comment form fields. Use add_filter('comment_form_default_fields', 'theme_comment_form_default_fields_callback') filter to modify comment form default fields. Use add_filter('comment_form_logged_in', 'theme_comment_form_logged_in_callback') filter to modify comment form logged in info. Use add_filter('comment_form_must_log_in', 'theme_comment_form_must_log_in_callback') filter to modify comment form must log in info. Use add_filter('comment_form_logged_in_after', 'theme_comment_form_logged_in_after_callback') filter to add content after logged in info. Use add_filter('comment_form_before', 'theme_comment_form_before_callback') filter to add content before comment form. Use add_filter('comment_form_after', 'theme_comment_form_after_callback') filter to add content after comment form. Use add_filter('comment_form_top', 'theme_comment_form_top_callback') filter to add content at top of comment form. Use add_filter('comment_form_bottom', 'theme_comment_form_bottom_callback') filter to add content at bottom of comment form. Use add_filter('comment_form_action', 'theme_comment_form_action_callback') filter to modify comment form action URL. Use add_filter('comment_form_method', 'theme_comment_form_method_callback') filter to modify comment form method. Use add_filter('comment_form_id_form', 'theme_comment_form_id_form_callback') filter to modify comment form ID. Use add_filter('comment_form_class_form', 'theme_comment_form_class_form_callback') filter to modify comment form class. Use add_filter('comment_form_id_submit', 'theme_comment_form_id_submit_callback') filter to modify comment submit button ID. Use add_filter('comment_form_class_submit', 'theme_comment_form_class_submit_callback') filter to modify comment submit button class. Use add_filter('comment_form_name_submit', 'theme_comment_form_name_submit_callback') filter to modify comment submit button name. Use add_filter('comment_form_label_submit', 'theme_comment_form_label_submit_callback') filter to modify comment submit button label. Use add_filter('comment_form_title_reply', 'theme_comment_form_title_reply_callback') filter to modify comment form title. Use add_filter('comment_form_title_reply_to', 'theme_comment_form_title_reply_to_callback') filter to modify reply comment form title. Use add_filter('comment_form_cancel_reply_link', 'theme_comment_form_cancel_reply_link_callback') filter to modify cancel reply link text. Use add_filter('comment_form_label_submit', 'theme_comment_form_label_submit_callback') filter to modify comment submit button label. Use add_filter('comment_form_submit_button', 'theme_comment_form_submit_button_callback') filter to modify comment submit button HTML. Use add_filter('comment_form_submit_field', 'theme_comment_form_submit_field_callback') filter to modify comment submit field HTML. Use add_filter('comment_form_fields', 'theme_comment_form_fields_callback') filter to modify comment form fields array. Use add_filter('comment_form_default_fields', 'theme_comment_form_default_fields_callback') filter to modify comment form default fields array. Use add_filter('comment_form_field_author', 'theme_comment_form_field_author_callback') filter to modify author field. Use add_filter('comment_form_field_email', 'theme_comment_form_field_email_callback') filter to modify email field. Use add_filter('comment_form_field_url', 'theme_comment_form_field_url_callback') filter to modify URL field. Use add_filter('comment_form_field_comment', 'theme_comment_form_field_comment_callback') filter to modify comment content field. Use add_filter('comment_form_field_cookies', 'theme_comment_form_field_cookies_callback') filter to modify cookie field. Use add_filter('comment_form_logged_in', 'theme_comment_form_logged_in_callback') filter to modify logged in info HTML. Use add_filter('comment_form_must_log_in', 'theme_comment_form_must_log_in_callback') filter to modify must log in info HTML. Use add_filter('comment_form_logged_in_after', 'theme_comment_form_logged_in_after_callback') filter to add HTML after logged in info. Use add_filter('comment_form_before', 'theme_comment_form_before_callback') filter to add HTML before comment form. Use add_filter('comment_form_after', 'theme_comment_form_after_callback') filter to add HTML after comment form. Use add_filter('comment_form_top', 'theme_comment_form_top_callback') filter to add HTML at top of comment form. Use add_filter('comment_form_bottom', 'theme_comment_form_bottom_callback') filter to add HTML at bottom of comment form. Use add_filter('comment_form_action', 'theme_comment_form_action_callback') filter to modify comment form action URL. Use add_filter('comment_form_method', 'theme_comment_form_method_callback') filter to modify comment form method. Use add_filter('comment_form_id_form', 'theme_comment_form_id_form_callback') filter to modify comment form ID. Use add_filter('comment_form_class_form', 'theme_comment_form_class_form_callback') filter to modify comment form class. Use add_filter('comment_form_id_submit', 'theme_comment_form_id_submit_callback') filter to modify comment submit button ID. Use add_filter('comment_form_class_submit', 'theme_comment_form_class_submit_callback') filter to modify comment submit button class. Use add_filter('comment_form_name_submit', 'theme_comment_form_name_submit_callback') filter to modify comment submit button name. Use add_filter('comment_form_label_submit', 'theme_comment_form_label_submit_callback') filter to modify comment submit button label. Use add_filter('comment_form_title_reply', 'theme_comment_form_title_reply_callback') filter to modify comment form title. Use add_filter('comment_form_title_reply_to', 'theme_comment_form_title_reply_to_callback') filter to modify reply comment form title. Use add_filter('comment_form_cancel_reply_link', 'theme_comment_form_cancel_reply_link_callback') filter to modify cancel reply link text. Use add_filter('comment_form_submit_button', 'theme_comment_form_submit_button_callback') filter to modify comment submit button HTML. Use add_filter('comment_form_submit_field', 'theme_comment_form_submit_field_callback') filter to modify comment submit field HTML. Use add_filter('comment_form_fields', 'theme_comment_form_fields_callback') filter to modify comment form fields array. Use add_filter('comment_form_default_fields', 'theme_comment_form_default_fields_callback') filter to modify comment form default fields array. Use add_filter('comment_form_field_author', 'theme_comment_form_field_author_callback') filter to modify author field. Use add_filter('comment_form_field_email', 'theme_comment_form_field_email_callback') filter to modify email field. Use add_filter('comment_form_field_url', 'theme_comment_form_field_url_callback') filter to modify URL field. Use add_filter('comment_form_field_comment', 'theme_comment_form_field_comment_callback') filter to modify comment content field. Use add_filter('comment_form_field_cookies', 'theme_comment_form_field_cookies_callback') filter to modify cookie field. Use add_filter('comment_form_logged_in', 'theme_comment_form_logged_in_callback') filter to modify logged in info HTML. Use add_filter('comment_form_must_log_in', 'theme_comment_form_must_log_in_callback') filter to modify must log in info HTML. Use add_filter('comment_form_logged_in_after', 'theme_comment_form_logged_in_after_callback') filter to add HTML after logged in info. Use add_filter('comment_form_before', 'theme_comment_form_before_callback') filter to add HTML before comment form. Use add_filter('comment_form_after', 'theme_comment_form_after_callback') filter to add HTML after comment form. Use add_filter('comment_form_top', 'theme_comment_form_top_callback') filter to add HTML at top of comment form. Use add_filter('comment_form_bottom', 'theme_comment_form_bottom_callback') filter to add HTML at bottom of comment form. Use add_filter('comment_form_action', 'theme_comment_form_action_callback') filter to modify comment form action URL. Use add_filter('comment_form_method', 'theme_comment_form_method_callback') filter to modify comment form method. Use add_filter('comment_form_id_form', 'theme_comment_form_id_form_callback') filter to modify comment form ID. Use add_filter('comment_form_class_form', 'theme_comment_form_class_form_callback') filter to modify comment form class. Use add_filter('comment_form_id_submit', 'theme_comment_form_id_submit_callback') filter to modify comment submit button ID. Use add_filter('comment_form_class_submit', 'theme_comment_form_class_submit_callback') filter to modify comment submit button class. Use add_filter('comment_form_name_submit', 'theme_comment_form_name_submit_callback') filter to modify comment submit button name. Use add_filter('comment_form_label_submit', 'theme_comment_form_label_submit_callback') filter to modify comment submit button label. Use add_filter('comment_form_title_reply', 'theme_comment_form_title_reply_callback') filter to modify comment form title. Use add_filter('comment_form_title_reply_to', 'theme_comment_form_title_reply_to_callback') filter to modify reply comment form title. Use add_filter('comment_form_cancel_reply_link', 'theme_comment_form_cancel_reply_link_callback') filter to modify cancel reply link text.