<div <?php post_class() ?> id="post-<?php the_ID(); ?>">
   <!-- Post stuff -->
</div>

I was in a circumstance where I wanted to add an additional class to what that was outputting. My thought process went like this:

1. Arg. I guess I’m going to have to write a function to filter the output of that function. Let’s begin the research.
2. Wait. I bet that function has an alternate that returns a string rather than echos it. I’ll just use that, and echo it out along with my own custom class.
3. Double wait. I bet those smart WordPress folks have thought of this…

1. Filtering

This kind of thing is generally beyond my comfort zone, but you’d basically write your own little function which takes in what the output would have been, add you own stuff to it, then return the new value.

Then call the new function with a filter in your functions.php file:

add_filter('post_class','add_my_stuff');

2. Output exiting classes, then add yours

Like most functions that echo stuff, there is a version that doesn’t echo stuff, in which “get_” prefaces the function name. I didn’t even have to look it up I just knew there would be. Since it’s a list, this function returns an array, so to output them, you’ll need to loop through the array:

<div class="<?php $allClasses = get_post_class(); foreach ($allClasses as $class) { echo $class . " "; } ?> group" id="post-<?php the_ID(); ?>">

The cool part here is that is doesn’t add “class=” in the output, you can do that yourself, meaning that I can add my own classes after the output is finished.

3. Built in…

Of course, as it turns out, this is far easier than #2 made it out to be. To add additional classes, just pass them as a parameter!

<?php post_class('special'); ?>

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