如何在PHP中使用fopen()将密码写入txt

I want to have this script which receives a POST from another PHP script to append the contents (username in this case) to a file called passwords.txt which already exists in the same directory as this script. The print is there to check that the POST is successful (it is). Here's the code:

</html>
<body>
   <?php
      $indo=$_POST["username"];
      $handle=fopen('/usr/share/phpMyAdmin/passwords.txt',"a");
      print($_POST["username"]);
      fwrite($handle, $indo);
      fclose($handle);
   ?>
</body>
</html>