Anonymous comments per post

anonymous-comments-per-post.php

<?php
/**
 * Plugin Name: Anonymous comments per post
 * Plugin URI:  http://wordpress.stackexchange.com/q/73788/73
 * Version:     2012.11.24
 * Author:      Fuxia Scholz
 * Author URI:  https://fuxia.me
 * Licence:     MIT
 * License URI: http://opensource.org/licenses/MIT
 */

add_action( 'post_comment_status_meta_box-options', 'acpp_checkbox' );
/**
 * Print a checkbox into the comment status metabox.
 *
 * @wp-hook post_comment_status_meta_box-options
 * @param   object $post
 * @return  void
 */
function acpp_checkbox( $post )
{
	$key = '_allow_anonymous_comments';
	$current = get_post_meta( $post->ID, $key, TRUE );

	printf(
		'<br /><label for="%1$s">
		<input type="checkbox" id="%1$s" name="%1$s" class="selectit" %2$s/> %3$s
		</label>',
		$key,
		checked( 1, $current, FALSE ),
		apply_filters( 'acpp_metabox_label', 'Allow anonymous comments.' )
	);
}

add_action( 'save_post', 'acpp_save' );
/**
 * Save the checkbox value as number
 *
 * @wp-hook save_post
 * @param   int $post_id
 * @param   object $post
 * @return  void
 */
function acpp_save( $post_id, $post )
{
	// AJAX autosave
	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
		return;

	// Some other POST request
	if ( ! isset ( $_POST['post_type'] ) )
		return;

	// Missing capability
	if ( ! current_user_can( 'edit_' . $_POST['post_type'], $post_id ) )
		return;

	$key = '_allow_anonymous_comments';

	// Checkbox successfully clicked
	if ( isset ( $_POST[ $key ] ) and 'on' === strtolower( $_POST[ $key ] ) )
		return update_post_meta( $post_id, $key, 1 );

	// Checkbox deselected
	delete_post_meta( $post_id, $key );
}

add_filter( 'pre_option_comment_registration', 'acpp_comment_reg_filter' );
/**
 * Trick the registration checks on front-end
 *
 * Important: If we return FALSE, the filter will be effectively ignored.
 * It has to be any other value.
 *
 * @wp-hook pre_option_comment_registration
 * @return bool|int
 */
function acpp_comment_reg_filter()
{
	if ( is_admin() )
		return FALSE;

	$key     = '_allow_anonymous_comments';
	$post_id = 0;

	// Only available on wp-comments-post.php, not on regular post pages.
	if ( isset( $_POST['comment_post_ID'] ) )
		$post_id = (int) $_POST['comment_post_ID'];

	//
	$post = get_post( $post_id );
	$open = get_post_meta( $post->ID, $key, TRUE );

	if ( 1 == $open )
		return 0;

	return FALSE;
}