How to get all unique categories for posts in loop?

Question

I'm looking to get a plain list of the unique categories in a custom loop (only categories for the posts in the loop). I've been fumbling with some code for a bit and here's what I've got:

<?php
    $args = array(
        'post_status'=>'publish',
        'post_type'=>'post',
        'posts_per_page'=>-1
    );
    $get_posts = new WP_Query();

    $get_posts->query($args);
    if($get_posts->have_posts()) {

    $cats = array();
    while($get_posts->have_posts()) { $get_posts->the_post();

            $post_categories = wp_get_post_categories( get_the_ID() );
            $i = 0;
            foreach($post_categories as $c){
                $cat = get_category( $c );
                $cats[$i] = $cat->slug ;
                $i++;
            }

        } //endwhile
        $result = array_unique($cats);
       print_r($result);

    } //endif
    wp_reset_postdata();
?>

This will put each posts categories into an array and I can print that out. But I want to merge each posts array of categories, remove duplicates (so there's only one instance of each unique category), and then be able to print it out.

Essentially what I'm wanting to accomplish is to have a drop down for these categories to use for being able to sort by a particular category by refreshing the loop with that selected category. I can handle the rest, I just need to get this list of unique, unduplicated categories for all posts in a loop.

Anybody have any ideas?

Many thanks, Jonah

Answer 1

EDIT: Moved $i = 0 outside the loop. That should get you the full list of categories.

$get_posts = new WP_Query();
$i = 0;
$get_posts->query($args);
if($get_posts->have_posts()) {
    $cats = array();
    while($get_posts->have_posts()) { $get_posts->the_post();
        $post_categories = wp_get_post_categories( get_the_ID() );
        foreach($post_categories as $c){
            $cat = get_category( $c );
            $cats[$i] = $cat->slug ;
            $i++;
        }
    } //endwhile
    $result = array_unique($cats);
    print_r($result);

} //endif
wp_reset_postdata();

Basically it flattens the nested arrays in $cats - I think array_unique may have been discarding categories because they had an identical key.

Answer 2

EDIT

You are saving the categories in the array. This is very good. I am changing this block on code. See the modified code

    $cats = array();    
    while($get_posts->have_posts()) { $get_posts->the_post();

        $post_categories = wp_get_post_categories( get_the_ID() );

        foreach($post_categories as $c){
            $cat = get_category( $c );
            $cats[] = array( 'slug' => $cat->slug );
        }

    } //endwhile
    $result = array_unique($cats);
   print_r($result);

There have array_unique function. Using this function you can get unique value. See the tutorial here for better knowledge.