PHP class for threaded comments

I had to set up a simple threaded comments system in a PHP project yesterday and didn’t find any standout options. The presentation requirements were pretty simple and my only real code requirement was to use a single SQL query (using a self-referencing parent_id foreign key field) to fetch the comments. I didn’t want to mess with a full-on tree either (that would be overkill as I’d never need to walk up the tree, just needed to show all the comments for a single object).

What I found was either laughably pathetic or various Wordpress plug-ins that were even more hard to parse than the main Wordpress codebase.

I ended up writing a simple class that accomplishes it without getting too fancy. The main meat of the class is a recursively called method (Threaded_comment::print_parent()) that prints a comment and then its children. The class as I have shared below will need modification depending on how you use it, for my use I modified it to be a CodeIgniter library and output comments using a View file. Simple enough. It’s set-up as an example, not a full-on third party library. I am sure I’ll have to hack on it more, but hopefully it helps someone.

Update: Note that this code is not running the comments you see below–that’s powered by WordPress’ native comment feature (which also does threading). The code below is if you want to setup a threaded comment system in your own PHP application. You could certainly attempt to run it on a WordPress install, but there would be no point because the feature is already seamlessly built-in.

class Threaded_comments
{

    public $parents  = array();
    public $children = array();

    /**
     * @param array $comments
     */
    function __construct($comments)
    {
        foreach ($comments as $comment)
        {
            if ($comment['parent_id'] === NULL)
            {
                $this->parents[$comment['id']][] = $comment;
            }
            else
            {
                $this->children[$comment['parent_id']][] = $comment;
            }
        }
    }

    /**
     * @param array $comment
     * @param int $depth
     */
    private function format_comment($comment, $depth)
    {
        for ($depth; $depth > 0; $depth--)
        {
            echo "\t";
        }

        echo $comment['text'];
        echo "\n";
    }

    /**
     * @param array $comment
     * @param int $depth
     */
    private function print_parent($comment, $depth = 0)
    {
        foreach ($comment as $c)
        {
            $this->format_comment($c, $depth);

            if (isset($this->children[$c['id']]))
            {
                $this->print_parent($this->children[$c['id']], $depth + 1);
            }
        }
    }

    public function print_comments()
    {
        foreach ($this->parents as $c)
        {
            $this->print_parent($c);
        }
    }

}

Here’s the example usage with the data provided as an array. Remember that if your data is in another format you’ll have to modify the class.

$comments = array(  array('id'=>1, 'parent_id'=>NULL,   'text'=>'Parent'),
                    array('id'=>2, 'parent_id'=>1,      'text'=>'Child'),
                    array('id'=>3, 'parent_id'=>2,      'text'=>'Child Third level'),
                    array('id'=>4, 'parent_id'=>NULL,   'text'=>'Second Parent'),
                    array('id'=>5, 'parent_id'=>4,   'text'=>'Second Child')
                );

$threaded_comments = new Threaded_comments($comments);

$threaded_comments->print_comments();

Example Output:

Parent
	Child
		Child Third level
Second Parent
	Second Child