A Little About the setup_postdata() WordPress Function

Today I spent some minutes trying to get setup_postdata() to work. The problem I was having was that the function seemed to be setting the data for one post only. I was using it along with get_posts. If you look at the setup_postdata function documentation, you will find that it is really of not much help when it comes to finding out why it might not be working properly.

This is the situation I found myself in: I was developing a widget that takes the latest posts from the specified category and displays them. That is pretty easy to do. However, I hit an error when I was displaying the posts. The code looked like this:

function widget($args, $instance){
  //..some standard code
  $posts = get_posts($options);
  foreach($posts as $post){
    setup_postdata($post);
    //..some more code here
  }
  //..some more code here
}

I’ve got rid of the code that is not relevant to the situation.

That seemed to not work properly, and I got this output:

title1
title1
title1
title1

When I should have been getting this:

title1
title2
title3
title4

I went and checked the documentation of the functions involved and nothing seemed to be wrong. So I decided to look at the code of the function. I knew that the loop was setting the proper value for $post on each round. But for some reason setup_postdata was setting up the data for the same post over and over again.

It turns out I don’t really need to setup_postdata() at all, but I do it just to be sure. The functions I’m using to display the information are the_permalink, and the_title.

setup_postdata doesn’t really set up all the data related to a post. It sets only some of it, specifically the numeric ID, the author data, the day, the month, the current page, the content, whether the post is multi-page, the number of pages, and variable named $more that I currently am not sure what it is used for. This information is used by some functions like the_content.

The source of the problem was not really on setup_postdata, but on the_title and the_permalink. the_title, for example, relies on a function called get_the_title, which takes a single argument: a post id. This post id can be 0, which in fact is the default value. get_the_title then calls get_post, and passes the post id as parameter to that function. get_post, in turn, checks for that post id, if it is an empty value (zero is an empty value), then it uses the global $post, and that is where the problem was.

In order for functions like the_title to work properly when inside a function, you must declare a global post variable:

global $post;

you should do that, preferably at the top of your function. This overwrites the current global $post with the post that you are trying to access information from.

So, as it turns out, setup_postdata is a function that only sets some global variables that can be used by other functions like the_content. Keep in mind that you don’t always have to use setup_postdata, and if you do use it, always consider running wp_reset_postdata(); when you are done.

And finally, remember that functions like the_title, the_permalink, the_content, and so on, rely on the global $post. This means that you should always declare a global $post inside any function that uses those functions mentioned earlier. When looping through a set of posts that you got from functions like get_posts, always name the current item $post:

foreach($posts as $post)//<-That $post is important! Do not use another name, or it won't work.

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