Important WordPress theme functions

Updated as on April 9, 2026

I have been working on WordPress for years and I still carry aroung a standard functions.php file while develop a new WordPress theme.

I am explaining here how to modernizing functions.php file so it will work seamlessly with WordPress 6 +.

These are the list of very common and useful features included here.

• Initial theme setup with navigation menu, featured image, custom image size, excerpt support
• Sidebar registration
• Image compression while uploading
• Responsive image fixing
• Enable sidebar widget
• Shorten post / page title by word limit
• Shorten excerpt by word limit
• Shorten content by word limit
• Strip images from content
• Display the content of a page by page ID
• Display the content of a page by page slug
• Anti spam email shortcode inside content editor
• Change default sender name and sender email address
• Remove WordPress version from Head
• Enqueue script and CSS
• Disable admin bar for all users but admins in the front end
• Disable default widgets

Add these codes in functions.php file of the active theme.

<?php
/**
 * Theme Setup
 */
function mytheme_setup() {

    // Navigation Menus
    register_nav_menus([
        'top-menu'    => __('Header Navigation', 'mytheme'),
        'bottom-menu' => __('Footer Navigation', 'mytheme'),
    ]);

    // Featured Images
    add_theme_support('post-thumbnails');

    // Custom Image Sizes
    add_image_size('blog_featured', 600, 300, true);

    // Add excerpt support to pages
    add_post_type_support('page', 'excerpt');
}
add_action('after_setup_theme', 'mytheme_setup');


/**
 * Sidebar Registration
 */
function mytheme_widgets_init() {
    register_sidebar([
        'name'          => __('Sidebar', 'mytheme'),
        'id'            => 'sidebar',
        'description'   => __('Main Sidebar', 'mytheme'),
        'before_widget' => '<div class="widget %2$s">',
        'after_widget'  => '</div>',
        'before_title'  => '<h3 class="widget-title">',
        'after_title'   => '</h3>',
    ]);
}
add_action('widgets_init', 'mytheme_widgets_init');


/**
 * Image Quality (Modern WebP/JPEG)
 */
add_filter('wp_editor_set_quality', function() {
    return 82;
});


/**
 * Remove width & height attributes (responsive images fix)
 */
function mytheme_remove_image_dimensions($html) {
    return preg_replace('/(width|height)="\d*"\s?/', '', $html);
}
add_filter('post_thumbnail_html', 'mytheme_remove_image_dimensions');
add_filter('image_send_to_editor', 'mytheme_remove_image_dimensions');


/**
 * Trim Title by Words
 */
function mytheme_short_title($length = 8, $after = '...') {
    $title = wp_strip_all_tags(get_the_title());
    $words = explode(' ', $title);

    if (count($words) > $length) {
        $words = array_slice($words, 0, $length);
        return implode(' ', $words) . $after;
    }

    return $title;
}


/**
 * Custom Excerpt Length
 */
function mytheme_excerpt($limit = 25) {
    return wp_trim_words(get_the_excerpt(), $limit, '...');
}


/**
 * Custom Content Trim
 */
function mytheme_content($limit = 25) {
    return wp_trim_words(
        wp_strip_all_tags(get_the_content()),
        $limit,
        '...'
    );
}


/**
 * Remove Images from Content
 */
function mytheme_strip_images($content) {
    return preg_replace('/<img[^>]+>/i', '', $content);
}
// Usage: add_filter('the_content', 'mytheme_strip_images');


/**
 * Get Page Content by ID
 */
function mytheme_get_page_content_by_id($page_id) {
    $page = get_post($page_id);

    if (!$page) return '';

    return apply_filters('the_content', $page->post_content);
}


/**
 * Get Page Content by Slug
 */
function mytheme_get_page_content_by_slug($slug) {
    $page = get_page_by_path($slug);

    if (!$page) return '';

    return apply_filters('the_content', $page->post_content);
}


/**
 * Email Shortcode [email]example@mail.com[/email]
 */
function mytheme_email_shortcode($atts, $content = null) {
    if (!$content) return '';

    $email = antispambot($content);

    return '<a href="' . esc_attr('mailto:' . $email) . '">' . esc_html($email) . '</a>';
}
add_shortcode('email', 'mytheme_email_shortcode');


/**
 * Mail Sender Customization
 */
add_filter('wp_mail_from', function() {
    return 'info@yourdomain.com';
});

add_filter('wp_mail_from_name', function() {
    return get_bloginfo('name');
});


/**
 * Remove WP Version
 */
add_filter('the_generator', '__return_empty_string');


/**
 * Enqueue Scripts & Styles
 */
function mytheme_enqueue_assets() {

    // CSS
    wp_enqueue_style(
        'mytheme-style',
        get_stylesheet_uri(),
        [],
        wp_get_theme()->get('Version')
    );

    // JS
    wp_enqueue_script(
        'mytheme-custom',
        get_template_directory_uri() . '/js/custom.js',
        ['jquery'],
        null,
        true
    );
}
add_action('wp_enqueue_scripts', 'mytheme_enqueue_assets');


/**
 * Disable Admin Bar for Non-Admins
 */
add_filter('show_admin_bar', function($show) {
    return current_user_can('administrator') ? $show : false;
});


/**
 * Remove Default Widgets (Optional)
 */
function mytheme_unregister_widgets() {
    unregister_widget('WP_Widget_Pages');
    unregister_widget('WP_Widget_Calendar');
    unregister_widget('WP_Widget_Archives');
    unregister_widget('WP_Widget_Meta');
    unregister_widget('WP_Widget_Search');
    unregister_widget('WP_Widget_Tag_Cloud');
}
// add_action('widgets_init', 'mytheme_unregister_widgets', 11);