PHP cannot read $_GET parameter when sending AJAX request

PHP $_GETsuperglobals are populated with data from the query string of the requested URL (regardless of the HTTP method used to make the request).

Here is the URL you use:

url: 'follow_user.php',

There is no query string at all.

You might expect it to copy the query string from the page hosting the JavaScript. It doesn't work that way, you need to provide the data explicitly.

a little:

You are inserting data into database. You should use a POST request for this (but it does), but you should pass the data in the request body (otherwise you're likely to accidentally create something that responds incorrectly to a GET request.

So first thing to do: change PHP to $_POSTreplace $_GET.

While editing, your PHP is vulnerable to SQL injection attacks . You need to learn how to protect their code.

OK, so next you need to get the data from the query string of the current page.

You can read it using JavaScript. This answer explains several ways.

Once you have the data, you can put it into the request:

var user_id = (new URL(url_string)).searchParams.get("user_id");
// See the answer linked above to see how this works and for compatibility with old browsers

$.ajax({
    type: 'POST',
    url: 'follow_user.php',
    data: { user_id: user_id },
    success: function(data) {
        alert('done');
    }
});

...remember, this puts it in the request body, so you need to use $_POSTinstead of reading $_GET.