PHP - Project Help

hello , Im working on a fictional web-site as a final project for my course.
Im having a little trouble with placing php includes in my page and also printing the total sum of my order.
also im not sure how or where to place the order now button in order.php'\.

here is my git , most of the necessery code if found in order.php

https://github.com/niksou/ProjectWeb

I need some help with the category part on a old project .

The problem what I have at this moment that, when I click on the category all categories are showing up on the category.php page itself.

What whI wanne have when I click Uncategorised that in the category.php page only post are showing that have been made in Uncategorised category

function get_posts() {
    $sql = "SELECT 
              `blog`.`post_id` AS `id`,
              `blog`.`post_title` AS `title`,
              LEFT(`blog`.`post_body`, 512) AS `preview`,
              `blog`.`post_user` AS `user`,
              `blog_categories`.`id` AS `category_id`,
              `blog_categories`.`name` AS `category_name`,
              DATE_FORMAT(`blog`.`post_date`,  '%d-%m-%Y %H:%i') AS `date`,
              `blog_comments`.`total_comments`,           
              DATE_FORMAT(`blog_comments`.`last_comment`, '%d-%m-%Y %H:%i') AS `last_comment`
            FROM `blog`
               INNER JOIN `blog_categories` ON `blog_categories`.`id` = `blog`.`cat_id`
            LEFT JOIN ( 
                SELECT 
                   `post_id`,
                   COUNT(`comment_id`) AS `total_comments`,
                   MAX(`comment_date`) AS `last_comment`
                FROM `blog_comments`
                GROUP BY `post_id`
              ) AS `blog_comments`
              ON `blog`.`post_id` = `blog_comments`.`post_id`
              ORDER BY `blog`.`post_date` DESC";

    $posts = mysql_query($sql);

    $rows = array();
    while (($row = mysql_fetch_assoc($posts)) !== false) {
        $rows[]= array(
            'id'             => $row['id'],
            'title'          => $row['title'],
            'preview'        => $row['preview'],
            'user'           => $row['user'],   
            'date'           => $row['date'],           
            'category_id'    => $row['category_id'],    
            'category_name'  => $row['category_name'],
            'total_comments' => ($row['total_comments'] === null) ? 0 : $row['total_comments'],
            'last_comments'  => ($row['last_comment'] === null) ? 'never' : $row['last_comment']                                                            
        );
    }
    
    return $rows;
}

function get_post($pid) {
    $pid = (int)$pid;

    $sql =   "SELECT
                `post_title` AS `title`,
                `post_body` AS `body`,
                `post_user` AS `user`,
                `blog_categories`.`id` AS `category_id`,
                `blog_categories`.`name` AS `category_name`,
                DATE_FORMAT(`post_date`, '%d-%m-%Y %H:%i') AS `date`
            FROM `blog` 
                INNER JOIN `blog_categories` ON `blog_categories`.`id` = `blog`.`cat_id`
            WHERE `post_id` = '$pid'";

    $post = mysql_query($sql);
    $post = mysql_fetch_assoc($post);

    $post['blog_comments'] = get_comments($pid);

    return $post;      
}

The old way was like this 

   if ( isset($cat_id)) {
      $cat_id = (int) $cat_id;
      $query .= " WHERE `blog_categories`.`id` = $cat_id";
   }  

Database 

blog_categories

id | name |

----------- 

4 Uncategorised

1 Youtube

blog 

post_id | cat_id | post_title | post_body | post_user | post_date 

-----------------------------------------------------------------------------------

4 | 1 | Youtube | Youtube link | Admin | 2020-09-13 16:10:53

Web server
Apache/2.2.21 (Win64) PHP/5.3.10
MySQL-client versie: mysqlnd 5.0.8-dev - 20102224 - $Revision: 321634 $
PHP uitbreiding: mysqli Documentatie

phpMyAdmin
Versie informatie: 3.4.10.1, meest recente versie: 5.0.2

I hope someone can help me with this problem