Handler for filters and actions for the comment form.

class.TTT_Comment_Form.php

<?php
/**
 * Handler for filters and actions for the comment form.
 *
 * comment_form(); in the theme’s comments.php and WP 3+ is required.
 *
 * @author  "Thomas Scholz" http://toscho.de
 * @version 1.2
 *
 */
class TTT_Comment_Form
{
	public
		// Name and e-mail required?
		$req
		// Current commentor, data from cookie.
	,	$commenter
		// May be free extended
	,	$options  = array (
			'textarea_first' => TRUE
		)
		// Placeholder for the textarea code.
	,	$textarea = ''
	;

	/**
	 * Sets default values and calls the filter and actions function.
	 *
	 * @param array $options
	 */
	public function __construct( $options = array () )
	{
		$this->req       = (bool) get_option( 'require_name_email' );
		$this->commenter = wp_get_current_commenter();
		$this->options   = array_merge($this->options, $options);
		$this->filters_and_actions();
	}

	/**
	 * Registers necessary filters and actions.
	 *
	 * @return void
	 */
	public function filters_and_actions()
	{
		add_filter(
			'comment_form_default_fields'
		,	array ( $this, 'filter_default_fields' )
		);
		add_filter(
			'comment_form_defaults'
		,	array ( $this, 'filter_defaults' )
		);

		// Plugin: Subscribe To "Double-Opt-In" Comments
		if ( function_exists('show_subscription_checkbox') )
		{
			add_action('comment_form_after', 'show_subscription_checkbox');
		}

		// Textarea above the other fields?
		if ( $this->options['textarea_first'] )
		{
			add_action( 'comment_form_top', array ( $this, 'print_textarea') );
		}

		// Additional container around the comment text.
		add_filter(
			'comment_text'
		,	array ( $this, 'surround_comment_text' )
		);
	}

	/**
	 * Sets an additional container around the comment text.
	 *
	 * @param  string $comment_text
	 * @return string
	 */
	public function surround_comment_text($comment_text)
	{
		$class = apply_filters('ttt_comment_surrounder', 'comment_text_block');
		return "<div class='$class'>$comment_text</div>";
	}

	/**
	 * @see $this->options['textarea_first']
	 * @return void
	 */
	public function print_textarea()
	{
		print apply_filters( 'comment_form_field_comment', $this->textarea);
		return;
	}

	/**
	 * Filter for single line text fields.
	 *
	 * @param  array $f Array of text fields from WordPress
	 * @return array
	 */
	public function filter_default_fields($f)
	{
		$req_mark = $this->req ? ' *' : '';
		// No, I don’t use WAI-ARIA.
		$req_attr = $this->req ? ' required' : '';

		$f['author'] = '<p><label for="author">Name'
			. $req_mark
			. '</label> '
			. '<input id="author" name="author" type="text" size="30" value="'
			. esc_attr( $this->commenter['comment_author'] ) . '"'
			. $req_attr
			. '></p>';
		$f['email'] = '<p><label for="email">E-Mail'
			. $req_mark . '</label> '
			. '<input id="email" name="email" type="email" value="'
			. esc_attr(  $this->commenter['comment_author_email'] ) . '" size="30"'
			. $req_attr . '></p>';
		$f['url'] =  '<p><label for="url">Website</label> '
			. '<input id="url" name="url" type="url" value="'
			. esc_attr( $this->commenter['comment_author_url'] )
			. '" size="30" /></p>';

		return $f;
	}

	/**
	 * Filters some additional strings.
	 * You probably want to overwrite this in your extended class.
	 *
	 * @param  array $d
	 * @return array
	 */
	public function filter_defaults($d)
	{
		// unset() throws an error, because WP doesn’t check for existence.
		$d['comment_notes_after']  = '';
		$d['label_submit']         = 'Absenden';

		$name                      = trim($this->commenter['comment_author']);

		// Personalization
		$d['title_reply']          = ( isset ( $name[1] )
									? "Hallo $name! " : '' )
									. 'Was sagst du dazu?';

		// Reminder for the required fields.
		$d['comment_notes_before'] = $this->req
			? '' : 'Nur das Textfeld ist Pflicht.';
		$d['comment_field']        = '<p><label for="comment">'
			. 'Kommentar</label><textarea id="comment" name="comment"'
			. ' cols="50" rows="15" required></textarea></p>';

		// Store the original textarea in a property and remove it here.
		if ( $this->options['textarea_first'] )
		{
			$this->textarea     = $d['comment_field'];
			$d['comment_field'] = '';
		}

		return $d;
	}
}