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<?php
/**
 * Customize wp_die so it doesn't look so cheesy
 */
function my_custom_die_handler($message, $title = '', $args = array()) {
    // Get the path to the custom die template
    $template = get_theme_root() . '/' . get_template() . '/custom_die.php';

    // If not an admin page, and if the custom die template exists
    if (!is_admin() && file_exists($template)) {
        // The default response code is 500
        $defaults = array('response' => 500);

        // Combine the args and defaults
        $r = wp_parse_args($args, $defaults);

        // Determine what type of error we are dealing with
        if (function_exists('is_wp_error') && is_wp_error($message)) {
            // If no title, try to make one
            if (empty($title)) {
                $error_data = $message->get_error_data();

                if (is_array($error_data) && isset($error_data['title']))
                    $title = $error_data['title'];
            }

            // Get the actual errors
            $errors = $message->get_error_messages();

            // Depending on how many errors there are, do something
            switch (count($errors)) {
                case 0 :
                    $message = '';
                    break;
                case 1 :
                    $message = '<li>' . $errors[0] . '</li>';
                    break;
                default :
                    $message = '
            <li>' . join("</li><li>", $errors) . '</li>
          ';
                    break;
            }
        } else if (is_string($message)) {
            $message = '<li>' . $message . '</li>';
        }

        /**
         * Since wp-comments-post.php does not allow error 
         * messages to be changed, the error message string 
         * is run through str_replace to customize.
         */
        $lame_messages = array(
            'Sorry, comments are closed for this item.',
            'please fill the required fields (name, email).',
            'please enter a valid email address.',
            'please type a comment.'
        );

        $replacements = array(
            'Sorry, comments are closed.',
            'All comment form fields are required. Please try again.',
            'Please enter a valid email address.',
            'No comment in your submission. Please try again.'
        );

        // Save the original error message for later comparison
        $pre_replacements = $message;

        // Do the comment form error message replacements
        $message = str_replace($lame_messages, $replacements, $message);

        if (
        // If the back link was set in the args
                ( isset($r['back_link']) && $r['back_link'] ) OR

                // Or if message identified as comment error
                $message !== $pre_replacements
        ) {
            // Show the back button
            $back_link = '<p>
                            <a href="javascript:history.back()" class="back">Back to Post</a>
                          </p>';
        }

        // If the title is empty, set it as "Error"
        if (empty($title))
            $title = 'Error';

        require_once( $template );

        die();
    }
    else {
        _default_wp_die_handler($message, $title, $args);
    }
}

// Intermediate function is necessary to customize wp_die
function swap_die_handlers() {
    return 'my_custom_die_handler';
}

add_filter('wp_die_handler', 'swap_die_handlers');