How to stream an image from PHP for tracking purposes

762 Views Asked by Volomike At 24 April 2025 at 10:19

With affiliate marketing and PHP, I know how to make a tracking pixel like this:

<img alt="" src="http://example.com/pixel.php?tracking=43233" />

Then, in pixel.php I do something like:

<?php

$sTrack = @ $_GET['tracking'];
$sTrack = urldecode($sTrack);
$sTrack = strip_tags($sTrack);
$sTrack = trim($sTrack);
// do something with sTrack here
header('Content-type: image/gif');
readfile('images/invisible.gif');

Trouble is, on this offer page I'm receiving cname and cemail as query params on the URL, and the customer is hosting on something that doesn't support PHP but does let me insert Javascript, and I CAN'T use jQuery in this case. Instead, I need to do this in 100% pure Javascript. So, I can't read $_GET to pick up the cname and cemail.

I know that location.search will give me those parameters. I just need how to change the IMG tag so that like on a load event I can pass the location.search on the end. And I need it to work across all the browsers since IE7 and up, as well as Opera, Safari, and Chrome.

Original Q&A

There are 2 best solutions below

doc_id On 03 February 2012 at 03:21 BEST ANSWER

You can just use the http://example.com/pixel.php as the img src, and then check the referral in that PHP script.

caleb On 02 February 2012 at 20:33

Use Javascript to set image source.

<body onload="document.getElementById('tracking_pixel').src='http://example.com/pixel.php'+location.search;">
    <img id="tracking_pixel" src="" />
</body>

Edit:

Yes, you can make it shorter by putting it in the img's onload. You can even drop the id and the this keyword. However, you cannot have the initial src to be empty, in fact, it has to be a valid image for it to trigger the onload event. The problem with this is that you incur the cost of a request over the network.

Here is the shortest I could muster, instead of repeating the url, I used += but that meant the onload handler had to be cleared.

<body>
    <img src="http://example.com/pixel.php" onload="onload=''; src+=location.search;" />
</body>

Alternatively, you can use data URI. Unfortunately, the data URI for just 1 pixel is pretty long.

<body>
    <img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAC0lEQVR42mNgAAIAAAUAAen63NgAAAAASUVORK5CYII=" onload="src='http://example.com/pixel.php'+location.search;" />
</body>

How to stream an image from PHP for tracking purposes

There are 2 best solutions below

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