Add Class to a div using a filter?

You could try using one of the following methods, I don’t think there is a specific way to do this other than these two options below.

Option 1

In your template file, add the post_class(); to your div like this:

<div id="your-div" <?php post_class(); ?> >

Then you can use post_class filter in your themes or plugins functions file, something like this:

// Add dynamic post class
    function royal_custom_class($classes) {
        global $post;
            $classes[] = 'your-class-name'; // Add your class however its generated here
            return $classes;
    }
    add_filter('post_class', 'royal_custom_class');

Option 2

Your other option, if the div was defined in your actual content then you could use the_content filter and replace and update it.

Something like this:

function updated_page_content( $content )
{
     return '<div class="custom_class">Whatever goes inside</div>'. $content;
}
add_filter( 'the_content', 'updated_page_content' );

References from Codex for these two methods

Post Class Filter – WordPress Codex

The Content Filter – WordPress Codex

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