Add Support for Custom Fields in WordPress Easily

Code

<?php
// Add support for custom fields in WordPress

// Hook into the 'init' action to register custom post type support for custom fields
function wp_dudecom_add_custom_fields_support() {
    // Add post type support for custom fields
    add_post_type_support('post', 'custom-fields');
    add_post_type_support('page', 'custom-fields');
    
    // Example: Add custom fields support for a custom post type 'product'
    add_post_type_support('product', 'custom-fields');
}
add_action('init', 'wp_dudecom_add_custom_fields_support');

// Function to display custom fields in the post editor
function wp_dudecom_display_custom_fields($post) {
    // Retrieve existing custom field value
    $custom_field_value = get_post_meta($post->ID, '_wp_dudecom_custom_field', true);
    
    // Display the custom field input box
    echo '<label for="wp_dudecom_custom_field">Custom Field:</label>';
    echo '<input type="text" id="wp_dudecom_custom_field" name="wp_dudecom_custom_field" value="' . esc_attr($custom_field_value) . '" />';
}

// Hook to add custom fields meta box
function wp_dudecom_add_custom_fields_meta_box() {
    add_meta_box(
        'wp_dudecom_custom_fields_meta_box',
        'Custom Fields',
        'wp_dudecom_display_custom_fields',
        ['post', 'page', 'product'],
        'normal',
        'high'
    );
}
add_action('add_meta_boxes', 'wp_dudecom_add_custom_fields_meta_box');

// Save custom field data when the post is saved
function wp_dudecom_save_custom_fields($post_id) {
    // Check if our nonce is set.
    if (!isset($_POST['wp_dudecom_custom_field_nonce'])) {
        return $post_id;
    }
    
    // Verify that the nonce is valid.
    if (!wp_verify_nonce($_POST['wp_dudecom_custom_field_nonce'], 'wp_dudecom_save_custom_fields')) {
        return $post_id;
    }
    
    // Check if the user has permission to edit the post.
    if (!current_user_can('edit_post', $post_id)) {
        return $post_id;
    }
    
    // Sanitize and save the custom field value
    $custom_field_value = sanitize_text_field($_POST['wp_dudecom_custom_field']);
    update_post_meta($post_id, '_wp_dudecom_custom_field', $custom_field_value);
}
add_action('save_post', 'wp_dudecom_save_custom_fields');

// Enqueue scripts and styles for the admin area
function wp_dudecom_enqueue_admin_scripts() {
    wp_enqueue_script('wp_dudecom_admin_script', get_template_directory_uri() . '/js/wp-dudecom-admin.js', ['jquery'], null, true);
    wp_enqueue_style('wp_dudecom_admin_style', get_template_directory_uri() . '/css/wp-dudecom-admin.css');
}
add_action('admin_enqueue_scripts', 'wp_dudecom_enqueue_admin_scripts');
?>