Show the posts under the same category - php

Question

I am quite new to wordpress and blogging. I am working on a blog portal that has several blogs but they all use common categories that are listed on the main page of the website. The problem is that whichever category I choose on the main page, it shows me the page with exactly same posts.
Here is my code:
<div class="category-holder list-center">
<h5>Kategoriat</h5>
<?php $site_url = get_bloginfo( 'wpurl' ); ?>
<a target="_blank" href="<?php $site_url; ?>/tags/?avain=<?php the_field('cat1'); ?>"><?php the_field('cat1'); ?></a><br/>
<a target="_blank" href="<?php $site_url; ?>/tags/?avain=<?php the_field('cat2'); ?>"><?php the_field('cat2'); ?></a><br/>
<a target="_blank" href="<?php $site_url; ?>/tags/?avain=<?php the_field('cat3'); ?>"><?php the_field('cat3'); ?></a><br/>
<a target="_blank" href="<?php $site_url; ?>/tags/?avain=<?php the_field('cat4'); ?>"><?php the_field('cat4'); ?></a><br/>
<a target="_blank" href="<?php $site_url; ?>/tags/?avain=<?php the_field('cat5'); ?>"><?php the_field('cat5'); ?></a>
</div>
Could you please suggest some PHP function or give me any tip what I should do, where I should look for a solution for this problem?I hope that you understand that I am just a beginner and I would like to learn more about programming.
Thank you in advance!
Update: I also have a functions.php file, the part of the code that might be relevant:
function tag_posts() {
global $wpdb;
$tag = $_GET['avain'];
$taxarr = array();
$postarr = array();
$blogs = $wpdb->get_results( "SELECT blog_id FROM {$wpdb->blogs} WHERE blog_id != {$wpdb->blogid} AND site_id = '{$wpdb->siteid}' AND spam = '0' AND deleted = '0' AND archived = '0'", ARRAY_A);
array_unshift($blogs, 1);
foreach($blogs as $blog) {
$wpdb->set_blog_id($blog[ 'blog_id' ]);
$tax_id = $wpdb->get_var("SELECT term_taxonomy_id FROM $wpdb->term_taxonomy WHERE term_id IN (SELECT term_id FROM $wpdb->terms WHERE name='$tag')");
$post_id = $wpdb->get_results("SELECT object_id FROM $wpdb->term_relationships WHERE term_taxonomy_id = $tax_id");
foreach ($post_id as $id) {
$postarr = $id->object_id;
}
$posts = $wpdb->get_results("SELECT * FROM {$wpdb->prefix}posts WHERE ID IN ($postarr)");
global $post;
foreach ($posts as $post):setup_postdata($post);
'<div class="post"><header class="post-header"><div class="date-holder"><span>'.the_time('F jS, Y').'</span></div>';
''.$post->post_title.'</header>';
'<div class="post-content">'.the_content().'</div></div>';
endforeach;
}
}

Answer 1

Please follow the steps. This may help you.
Create a new menu in Dashboard > Appearance > Menus.
Add categories in to the menu.
Use the function wp_nav_menu() to display the menu.
The category link in the menu displayed will direct you to the corresponding category page.
Please note that the category link direct you to the corresponding category page if and only if there exists category.php page in your themes folder.

Answer 2

Please, take a look at this loop here:
foreach ($post_id as $id) {
$postarr = $id->object_id;
}
$posts = $wpdb->get_results("SELECT * FROM {$wpdb->prefix}posts WHERE ID IN ($postarr)");
It seems to be that the expected value for $postarr at the moment you query for posts is an array having post IDs. However, when you do $postarr = $id->object_id; you are overwriting the variable assigning to it only the last ID in the result set and the result is an integer variable instead of an array.
So I would suggest something like this instead:
foreach ($post_id as $id) {
$postarr[] = $id->object_id;
}
$postsAsCommaSepString = implode( ',', $postarr );
$posts = $wpdb->get_results("SELECT * FROM {$wpdb->prefix}posts WHERE ID IN ($postsAsCommaSepString)");
First we are adding entries to the $postarr, note the brackets:
$postarr[] = $id->object_id;
Then, since we can't use an array variable as if it was a string, we need to create a string as a comma separated list of IDs to use in the SQL IN. For this, we implode the array into a string using commas as separator:
$postsAsCommaSepString = implode( ',', $postarr );
Finally, we use this new string variable in the query:
$posts = $wpdb->get_results("SELECT * FROM {$wpdb->prefix}posts WHERE ID IN ($postsAsCommaSepString)");
This should solve the problem of fetching the same posts every time.
EDIT: Bad query:
There is also a problem in this query:
$blogs = $wpdb->get_results( "SELECT blog_id FROM {$wpdb->blogs} WHERE blog_id != {$wpdb->blogid} AND site_id = '{$wpdb->siteid}' AND spam = '0' AND deleted = '0' AND archived = '0'", ARRAY_A);
In SQL, the not equal operator goes like this <> instead of !=. Please, see the corrected form below:
$blogs = $wpdb->get_results( "SELECT blog_id FROM {$wpdb->blogs} WHERE blog_id <> {$wpdb->blogid} AND site_id = '{$wpdb->siteid}' AND spam = '0' AND deleted = '0' AND archived = '0'", ARRAY_A);