我正在尝试使用HTML和PHP来加载文本文件并将其保存到服务器

desired sequence:

SaveButton
LoadButton When the "load button" is pressed, load a textfile from the server into a textbox.
Allow the user to add text
When the "save button" is pressed, save the textbox text to the server.

I am having issues with callbacks not executing.

I am worried about the callbacks, I know how to save a file.

<html>
<body>
<button id="1" onClick="reply_click(id)">Load</button>
<button id="2" onClick="reply_click(id)">Save</button>
<?php
function reply_click($clicked_id)
{
    echo "hello ";
}
?>
</body>
</html>

You're mixing JavaScript and PHP.

PHP is figured out and executed by the server. Then the web page is sent to the client. Then the user can interact with the page using JavaScript.

You'll have to change the onClick events to use AJAX that send a request to a PHP page.

You could use jQuery to handle AJAX requests without having to mess with low-level things like XMLHttpRequest.