Which action hook to use for function?

Question

I've created a custom category template to display the current category's child categories instead of posts. Child categories are displayed via a custom thumbnail, title and category description.

I have added a function to trim the category description to a predefined number of characters, but I'm not sure which action hook to use.

Here's the function:

function trim_text($input, $length, $ellipses = true, $strip_html = true) {
    //strip tags, if desired
    if ($strip_html) {
        $input = strip_tags($input);
    }

    //no need to trim, already shorter than trim length
    if (strlen($input) <= $length) {
        return $input;
    }

    //find last space within length
    $last_space = strrpos(substr($input, 0, $length), ' ');
    $trimmed_text = substr($input, 0, $last_space);

    //add ellipses (...)
    if ($ellipses) {
        $trimmed_text .= '...';
    }

    return $trimmed_text;
}

Answer 1

You don't need an action, but a filter, because you want to do something with the result of a function. The filter is, predictably, called category_description In this way (didn't test it though):

function wpse236947_trim_category_desc ( $desc, $cat_id ) {
    // do your thing
    return $desc;
}
add_filter( 'category_description', 'wpse236947_trim_category_desc' );

Answer 2

When asking the question I wasn't aware of the wp_trim_words() function which does exactly what I want. After removing the custom function from my functions.php file all I had to do was add <?php echo wp_trim_words( $child->description, 15, '...' ); ?> to my custom category template.