PHP: image in email

gistfile1.php

<?php
if(isset($_GET['uid']) && isset($_GET['mid'])){

 $mailid = (int)$_GET['mid'];
 $userid = (int)$_GET['uid'];

 $db = new mysqli_connect('databaseserver', 'databaseuser', 'databasepassword', 'databasename');
 $db->query("INSERT INTO mailreaders (mailid, userid) VALUES ($mailid, $userid)");

 $im=imagecreatetruecolor(1,1);
 $white=imagecolorallocate($im,255,255,255);
 imagefilledrectangle($im,0,0,1,1,$white);
 header("content-type: image/jpeg");
 imagejpeg($im);
 
}