I have a function in PHP, this function I want to access in HTML file. How can I do this?

Your HTML is viewed in a client (browser), and the function you want is in PHP on the server.

You basically have 2 options on how to do this:
1. Place your button within a FORM to be processed on the server
2. Call a javascript function which utilizes AJAX to call the server script for processing.

Reference:
html - How to call a PHP function on the click of a button - Stack Overflow[^]

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Updated for code samples

AjaxFunctions.php
This file has two parts:
_Part 1 sees if there is an action to perform and if so which to do
_Part 2 contains the various functions that can be called

<?php
    if (isset($_POST['action'])) {
        switch ($_POST['action']) {
            case 'PDF':
                DownloadPDF();
                break;
            case 'TXT':
                DownloadTxt();
                break;
        }
    }

    function DownloadPDF() {
        // Your PDF code
        exit;
    }

    function insert() {
        // Your TXT code
        exit;
    }
?>