Updating an old script to detect 404 errors

I used to run this script in my 404 page.
It was sending me an email with each 404 error. Sadly, it's not working any more, it's very old code. Can anyone help me to update it? Thanks in advance.

<?php
    $to = "email@domain.com";
# Gather visitor information
    $ip = getenv ("REMOTE_ADDR");                // IP Address
    $server_name = getenv ("SERVER_NAME");       // Server Name
    $request_uri = getenv ("REQUEST_URI");       // Requested URI
    $http_ref = getenv...

Code (markup):

Updating an old script to detect 404 errors

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