Custom WordPress Testimonials Metabox

testimonials.php

<?php 

// Add a new metabox to the testimonial post type
function add_testimonial_metabox() {
  // Use the WordPress function add_meta_box to define our metabox
  add_meta_box(
    'testimonial_metabox',         // Metabox ID
    'Testimonial Details',        // Metabox title
    'render_testimonial_metabox', // Callback function to render the metabox contents
    'testimonial',                // Post type where the metabox should appear
    'normal',                     // Metabox context (e.g. 'normal', 'side', or 'advanced')
    'high'                        // Metabox priority (e.g. 'high', 'core', 'default', or 'low')
  );
}

// Render the contents of the custom metabox
function render_testimonial_metabox($post) {
  // Use get_post_meta to retrieve any existing author and company data for this testimonial
  $testimonial_author = get_post_meta($post->ID, 'testimonial_author', true);
  $testimonial_company = get_post_meta($post->ID, 'testimonial_company', true);

  // Use wp_nonce_field to generate a nonce field for security purposes
  wp_nonce_field('save_testimonial_metabox', 'testimonial_metabox_nonce');

  // Output the HTML for the metabox
  ?>
  <p>
    <label for="testimonial_author">Author Name:</label><br>
    <input type="text" id="testimonial_author" name="testimonial_author" value="<?php echo esc_attr($testimonial_author); ?>">
  </p>
  <p>
    <label for="testimonial_company">Company:</label><br>
    <input type="text" id="testimonial_company" name="testimonial_company" value="<?php echo esc_attr($testimonial_company); ?>">
  </p>
  <?php
}

// Save the data entered in the custom metabox
function save_testimonial_metabox($post_id) {
  // Check that autosave is not in progress
  if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
    return;
  }

  // Verify that the nonce is valid
  if (!isset($_POST['testimonial_metabox_nonce']) || !wp_verify_nonce($_POST['testimonial_metabox_nonce'], 'save_testimonial_metabox')) {
    return;
  }

  // Check that the current user has permission to edit this post
  if (!current_user_can('edit_post', $post_id)) {
    return;
  }

  // Sanitize and validate the author and company data entered in the metabox
  $author_name = isset($_POST['testimonial_author']) ? sanitize_text_field($_POST['testimonial_author']) : '';
  $company_name = isset($_POST['testimonial_company']) ? sanitize_text_field($_POST['testimonial_company']) : '';

  if (empty($author_name) || empty($company_name)) {
    // Display an error message if either field is empty
    add_action('admin_notices', function() {
      ?>
      <div class="notice notice-error">
        <p><?php _e('Both Author Name and Company fields are required.', 'my_textdomain'); ?></p>
      </div>
      <?php
    });
    return;
  }

  // Update the post meta with the sanitized author and company data entered in the metabox
  update_post_meta($post_id, 'testimonial_author', $author_name);
  update_post_meta($post_id, 'testimonial_company', $company_name);
}

// Hook our add_testimonial_metabox and save_testimonial_metabox functions into WordPress
add_action('add_meta_boxes_testimonial', 'add_testimonial_metabox');
add_action('save_post_testimonial', 'save_testimonial_metabox');

----
  
/* To add the metabox to an existing custom post type, you'll need to modify the code a bit. 
  Here's an example of how to add the metabox to a custom post type named "my_custom_post_type": */

function my_custom_metabox() {
  add_meta_box(
    'testimonial_metabox',
    'Testimonial Details',
    'render_testimonial_metabox',
    'my_custom_post_type', // Change this to your custom post type slug
    'normal',
    'high'
  );
}

function render_testimonial_metabox( $post ) {
  // Add your metabox fields here
}

add_action( 'add_meta_boxes', 'my_custom_metabox' );

/* You'll want to replace "my_custom_post_type" with the slug of your existing custom post type, and update the render_testimonial_metabox function to include the fields you want to display in the metabox.
Once you've made these changes, simply add the modified code to your theme's functions.php file or a plugin and save the changes. The metabox should now be added to your existing custom post type. */