How to separate categories with commas?

Question

I have this function that displays the specified hierarchy level category of a post. For example <?php display_cat_level(0,true); ?> will display the top/parent category, while <?php display_cat_level(2,true); ?> will display the second child category.

If the post has more than one category in any given level then the categories will be displayed inline with no spaces or commas between them like this cat1acat1bcat1c instead of cat1a, cat1b, cat1c. How would I update the function to comma separate multiple categories displayed?

function get_level($category, $level = 0)
{
    if ($category->parent == 0) {
        return $level;
    } else {
        $level++;
        $category = get_term( $category->parent );
        return get_level($category, $level);
    }
}

function display_cat_level( $level = 0 , $link=false){

    $cats = get_the_terms( null, 'category' );
    if( $cats ){
        foreach($cats as $cat){
            $current_cat_level = get_level($cat);
            if( $current_cat_level  == $level ){
                if($link==true) {
                    echo '<a href="'.get_term_link( $cat->term_id ).'">'.$cat->name."</a>";
                } else {
                    echo $cat->name."";
                }
            }
        }
    }
}

Answer 1

All you have to do is to track if you already echoed some category and add a comma in such case:

function get_level($category, $level = 0) {
    if ($category->parent == 0) {
        return $level;
    }

    $category = get_term( $category->parent );
    return get_level( $category, $level + 1 );
}

function display_cat_level( $level = 0, $link = false) {
    $cats = get_the_terms( null, 'category' );
    $echoed = 0;

    if ( $cats ) {
        foreach ( $cats as $cat ) {
            $current_cat_level = get_level( $cat );
            if ( $current_cat_level  == $level ) {
                if ( $echoed ) {
                    echo ', ';
                }
                if ( true == $link ) {
                    echo '<a href="' . get_term_link( $cat->term_id ) . '">' . esc_html( $cat->name ) . "</a>";
                } else {
                    echo esc_html( $cat->name );
                }
                $echoed++;
            }
        }
    }
}

PS. I've also simplified your get_level function a little bit and added some html escaping in display_cat_level.