How to call a PHP function from Javascript in WordPress

I have a very simple plugin with a javascript file and a PHP file. I want to call the PHP file from my javascript code and get the output. The javascript functions is something like the following:

function img_upload(){

        var ajax = new XMLHttpRequest();           
        ajax.open('GET', 'http://My_Domain_Name.com/wp-content/plugins/My_Plugin/auth.php', false);
        ajax.send();

    if (ajax.status === 200) {

      console.log(ajax.responseText);

    }

and the PHP file which returns the results:

<?php
$token=Get_Token();
echo $token;
function Get_Token()
{
   //Do some stuff
   return $token;
}
?>

both files (auth.php and myjs.js) are in the root directory of the plugin.

/home/My_Username/public_html/wp-content/plugins/My_Plugin

If I use the domain name I can call the php file in ajax.open() and get the results, but I know this is not the right way to do that. How can I call the php file inside my javascrip code via ajax.open(‘path_to_php’) in WordPress properly without indicating the domain name?

Here’s an example:

Use this sample JavaScript code:

jQuery(document).on('click', '.some-element', function(e){
    var ipc = jQuery(this).data('collection-id');
    jQuery('.some-other-element').show();

    jQuery.ajax({
        method: 'post',
        url: ipAjaxVar.ajaxurl,
        data: {
            collection_id: ipc,
            action: 'my_function',
        }
    }).done(function(msg) {
        // Do something when done
    });

    e.preventDefault();
});

PHP (include the function in your plugin, do not use a separate file):

// Include the JavaScript above in your plugin
wp_enqueue_script('main', plugins_url('js/jquery.main.js', __FILE__), array('jquery'), '', true);

wp_localize_script('main', 'ipAjaxVar', array(
    'ajaxurl' => admin_url('admin-ajax.php')
));

add_action('wp_ajax_my_function', 'my_function');

UPDATE:

Add the PHP code to your main plugin file. Create a JavaScript file – js/jquery.main.js – and add the code above. That should do the trick.