Add Custom Field Validation in WordPress for Required Fields

Code

<?php
// Hook into the 'save_post' action to validate custom fields
add_action('save_post', 'wp_dudecom_validate_custom_fields');

function wp_dudecom_validate_custom_fields($post_id) {
    // Check if our nonce is set.
    if (!isset($_POST['wp_dudecom_custom_fields_nonce'])) {
        return $post_id;
    }

    // Verify that the nonce is valid.
    if (!wp_verify_nonce($_POST['wp_dudecom_custom_fields_nonce'], 'wp_dudecom_custom_fields')) {
        return $post_id;
    }

    // If this is an autosave, our form has not been submitted, so we don't want to do anything.
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
        return $post_id;
    }

    // Check the user's permissions.
    if ('page' == $_POST['post_type']) {
        if (!current_user_can('edit_page', $post_id)) {
            return $post_id;
        }
    } else {
        if (!current_user_can('edit_post', $post_id)) {
            return $post_id;
        }
    }

    // Validate required fields
    if (empty($_POST['wp_dudecom_custom_field'])) {
        // Add an error message
        add_filter('redirect_post_location', function($location) {
            return add_query_arg('wp_dudecom_validation_error', '1', $location);
        });
        return $post_id;
    }

    // Sanitize and save the custom field
    $custom_field_value = sanitize_text_field($_POST['wp_dudecom_custom_field']);
    update_post_meta($post_id, '_wp_dudecom_custom_field', $custom_field_value);
}

// Display an error message if validation fails
add_action('admin_notices', 'wp_dudecom_custom_field_error_notice');

function wp_dudecom_custom_field_error_notice() {
    if (isset($_GET['wp_dudecom_validation_error'])) {
        echo '<div class="error"><p>' . __('Custom field is required.', 'textdomain') . '</p></div>';
    }
}

// Add a meta box to the post editing screen
add_action('add_meta_boxes', 'wp_dudecom_add_custom_meta_box');

function wp_dudecom_add_custom_meta_box() {
    add_meta_box(
        'wp_dudecom_custom_meta_box',
        __('Custom Field', 'textdomain'),
        'wp_dudecom_custom_meta_box_callback',
        'post',
        'normal',
        'high'
    );
}

function wp_dudecom_custom_meta_box_callback($post) {
    // Add a nonce field so we can check for it later.
    wp_nonce_field('wp_dudecom_custom_fields', 'wp_dudecom_custom_fields_nonce');

    // Retrieve an existing value from the database.
    $value = get_post_meta($post->ID, '_wp_dudecom_custom_field', true);

    // Display the form, using the current value.
    echo '<label for="wp_dudecom_custom_field">';
    _e('Enter your custom field:', 'textdomain');
    echo '</label> ';
    echo '<input type="text" id="wp_dudecom_custom_field" name="wp_dudecom_custom_field" value="' . esc_attr($value) . '" size="25" />';
}
?>