将ajax留言簿保存到文件

I have tried with fuction fopen, fwrite but I still can't make the it possible I want to save my ajax guest book into txt file, please help me. This is the index.php

<?php
error_reporting(E_ALL^E_NOTICE);

include "comment.class.php";

$comments = array();

foreach($comments as $c){
    echo $c->markup();
} 

?>

and it is the comment.class.php

<?php

class Comment
{
    private $data = array();

    public function __construct($row)
    {
        /*
        /   konstruktor
        */

        $this->data = $row;
    }

    public function markup()
    {
        /*
        /   method untuk comment
        */

        //alias untuk &$this->data
        $d = &$this->data;

        $link_open = '';
        $link_close = '';

        if($d['url']){


            $link_open = '<a href="'.$d['url'].'">';
            $link_close =  '</a>';
        }

        $d['dt'] = strtotime($d['dt']);
        $url = 'http://'.dirname($_SERVER['SERVER_NAME'].$_SERVER["REQUEST_URI"]).'/img/default_avatar.gif';

        return '

            <div class="comment">
                <div class="avatar">
                    '.$link_open.'
                    '.$link_close.'
                </div>

                <div class="name">'.$link_open.$d['name'].$link_close.'</div>
                <div class="date" title="Added at '.date('H:i \o
 d M Y',$d['dt']).'">'.date('d M Y',$d['dt']).'</div>
                <p>'.$d['body'].'</p>
            </div>
        ';
    }

I want to save the comment body into chat.txt

The following snippet will append the comments into the file instead of echoing them. See file_put_contents manual for details.

foreach($comments as $c){
    file_put_contents( 'chat.txt',  $c->markup(), FILE_APPEND );
}