Can'；t使用PHPMAC（Xampp）打开.txt文件

下面是代码给我的错误消息：

警告：fopen（users.txt）：无法打开流：第79行/opt/lampp/htdocs/assignment3/form.php中的权限被拒绝 无法打开文件

这是我的密码：

$file = fopen("users.txt", "a") or die("Unable to open file.");

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我刚开始用mac编写代码，所以我对它不太了解。提前感谢您的帮助

运行此代码以轻松找到正确的文件夹

<?php

$txt = 'users.txt';

echo '<br> your php file is here : ' . __FILE__;

echo '<br> your text file should exists in : ' . __DIR__ . '/' . $txt;

echo '<br> your text file '.( file_exists($txt)?'exists':'not exists' );

echo '<br> your text file is '.( is_readable($txt)?'readable':'not readable' );

echo '<br> your text file is '.( is_writable($txt)?'writable':'not writable' );

echo '<br> your dir is '.(
        is_writable(__DIR__)?
            'writable, then you can create new files':
            'not writable, then you can not create any files'
    );