WordPress get all category ids

To get all the category id’s from current wordpress blog you have to use following function which returns an array of all the category id’s

<?php $all_category_ids=get_all_category_ids() ?>

after that you can fetch to retrieve all the category ids. even you can get category name for each category.

<?php
$category_ids = get_all_category_ids();
foreach($category_ids as $cat_id) {
  $cat_name = get_cat_name($cat_id);
  echo $cat_id . ': ' . $cat_name.'<br />';
}
?> 

Credits

WordPress Codex
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