查询以选择与当前类似的帖子

我有一个表posts，其中包含以下行：

post_id
post_title
post_image
post_body
category

存储类别的第二表category

cat_id
cat_name

当我点击帖子1时，它在新页面post.php?post_id=1中打开。。我现在想做的是在新页面上只发布来自同一类别的类似帖子。这是我在单页上的代码

function truncate–s($text, $chars = 80) {
    $text = $text." ";
    $text = substr($text,0,$chars);
    $text = substr($text,0,strrpos($text,' '));
    $text = $text." ...";
    return $text;
}
if(isset($_GET['post_id']) && is_numeric($_GET['post_id'])){
                $post_id = $_GET['post_id'];
    $query = $pdo->query("SELECT * FROM posts WHERE post_id = $post_id");
            foreach ($query as $row) {
                    echo '<div class="col-lg-8 col-md-8 col-sm-12 col-xs-12">
                            <div class="portfolio-slider-wrapper">
                                <ul id="portfolio-slider">
                                <h4>'.$row['post_title'].'</h4>
                                <hr>
                                    <li><img src="'.$row['post_image'].'" alt="" class="img-responsive"/> </li>
                                </ul>
                            </div>
                        <div class="blog-desc">
                            <ul class="post-meta-links list-inline">
                                <li><a href="#"><span> <i class="fa fa-calendar"></i></span>'.$row['post_date'].'</a></li>                                    
                            </ul>
                    <p>
                        '.$row['post_body'].'
                    </p>
            </div>
  }
  echo ' <div class="clearfix"></div>
        <div class="related-post">
        <h4>Related</h4>
        <hr>';
         $query1 = $pdo->query("SELECT * FROM posts INNER JOIN category ON posts.category=category.cat_id LIMIT 3");             
        foreach ($query1 as $post) {                  
         echo '<div class="col-md-4 col-sm-4">
                 <div class="rel-post">
                    <a href="#">
                        <img src="'.$post['post_image'].'" alt="" lass="img-responsive">
                        <div class="caption">
                            <h4>'.$post['post_title'].'</h4>
                           <p>'.truncate–s($post['post_body']).'</p>
                        </div>
                    </a>
                 </div>
                </div>';
            }          

目前，当我打开一些帖子时，不管ID是什么，它总是从数据库表中加载前3个帖子。

如何加载相同类别的帖子？