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The two functions below are designed to return a list of posts that share the same category as the current post being viewed in single.php

However, when I var_dump on $cat (should be a list of the categories assigned to the current post), I'm not getting what I'm expecting. Do I need to deserialize the array before passing it to the get_posts query?

function get_cats()
$post_cats= array();
$categories = get_the_category();
foreach($categories as $cat){
array_push($post_cats, $cat->cat_ID);
return $post_cats;

//get related posts by category
function ce4_get_related_by_category()
global $post;
$cat = implode(',',get_cats());
$myqueryCurrent = new WP_Query();
$myqueryCurrent->query(array('cat' => "$cat,-$catHidden",'post__not_in' => get_option('sticky_posts')));
$totalpostcount = $myqueryCurrent->found_posts;
if($totalpostcount > 0)
        echo "<ul>";
        $myposts = get_posts(array('cat' => "$cat,-$catHidden",'numberposts' => $cb2_current_count));
        foreach($myposts as $idx=>$post) 
            <li><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a><?php the_excerpt(); ?></li>
        echo "</ul>";
share|improve this question
up vote 2 down vote accepted

You can use get_the_category()

It will return an array of category IDs belonging to the current post.

$post_cats= array();
$categories = get_the_category();
foreach($categories as $cat) :
array_push($post_cats, $cat->cat_id);

Then the $post_cats array will have a list of all the ids.

share|improve this answer
When I var_dump(get_the_category($post->ID)); it appears as if I'm getting the entire category objects. Can I pass an argument to limit the response to a list of cat_id's? – Scott B Jan 19 '11 at 15:40
You can't limit the response to only ids, but you could try something like this to get an array of only the IDs. (check my answer, edited) – Pippin Jan 19 '11 at 15:43
Thanks Pippin, I've updated my question with your suggestions. I'm getting array(1) { [0]=> NULL } when I var_dump($cat). I'm expecting '10,7' – Scott B Jan 19 '11 at 16:00
Should be $cat->cat_ID in Loop. – Rarst Jan 19 '11 at 16:13

You can fetch array of IDs alone like this with wp_get_object_terms():

wp_get_object_terms($id, 'category', array('fields' => 'ids'))

Note that get_the_category() caches results and getting IDs from its return might actually be more efficient than fetching IDs separately with this.

share|improve this answer
Thanks Rarst, do I need to deserialize the array before passing it to $myqueryCurrent and $myposts? – Scott B Jan 19 '11 at 16:24
@Scott B according to your current code you will need to loop through array to negate values and add them to string. I think it would be simpler to use category__not_in instead of cat and just pass array of IDs. – Rarst Jan 19 '11 at 16:27
I've edited the code. I just needed to run the implode method on the variable. It seems to work. I just need to remove the current post from the output loop. – Scott B Jan 19 '11 at 16:36
@Scott B ah, I misunderstood there slightly, thought you want to exclude these rather than include. Yep, in such case implode will do just fine. – Rarst Jan 19 '11 at 16:38

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