title: Problems with tracking pixels and Gmail proxy
body: <p>I am trying to implement a custom Tracking Pixel for Emails sent out from wordpress. </p>
  
  <p>Thanks to these post:</p>
  
  <p><a href="https://stackoverflow.com/questions/5448381/tracking-email-with-php-and-image">Tracking email with PHP and image</a></p>
  
  <p><a href="https://stackoverflow.com/questions/14000277/tracking-email-opens-with-a-real-image">Tracking email opens with a real image</a></p>
  
  <p>and especially</p>
  
  <p><a href="http://www.phpdevtips.com/2013/06/email-open-tracking-with-php-and-mysql/" rel="noreferrer">http://www.phpdevtips.com/2013/06/email-open-tracking-with-php-and-mysql/</a></p>
  
  <p>I was able to implement the core idea. </p>
  
  <p>The email loads the tracking pixel via 
          <code>&lt;img src="https://www.example.com/tracking.php?order_id=1" width="100" height="100" /&gt;</code></p>
  
  <p>and in the tracking.php </p>
  
  <pre><code>$graphic_http =  'https://www.example.com/GIF-example.gif';
  
  header('Content-Type: image/gif');
  readfile( $graphic_http );
  </code></pre>
  
  <p>Opening the tracking.php file in a browser opens up the gif image for download.</p>
  
  <p>However the Tracking pixel/Tracking image doesn't show up in the Gmail Email. There is only a broken image logo and when I click to show the image this link is opened</p>
  
  <p><a href="https://ci5.googleusercontent.com/proxy/l2xUKFGnNFKm64zEYmJhOcUmEJm15w9MC1txRRF01tpKlcL3t3O16aMJgbYQkucBySV0xV2T0EsCwikOAC0Z4em6uPzSs38lkHrYBvosRRAk14EfPoEXqC5JdLxRm8ToZmGSQqt_RwHCaBE_3uLgQDVEB05Rdtkq-Xzuw30=s0-d-e1-ft#https://www.example.com/tracking.php?order_id=1" rel="noreferrer">https://ci5.googleusercontent.com/proxy/l2xUKFGnNFKm64zEYmJhOcUmEJm15w9MC1txRRF01tpKlcL3t3O16aMJgbYQkucBySV0xV2T0EsCwikOAC0Z4em6uPzSs38lkHrYBvosRRAk14EfPoEXqC5JdLxRm8ToZmGSQqt_RwHCaBE_3uLgQDVEB05Rdtkq-Xzuw30=s0-d-e1-ft#https://www.example.com/tracking.php?order_id=1</a></p>
  
  <p>which states a Google 404:</p>
  
  <p>Google 404. That’s an error.</p>
  
  <p>The requested URL /proxy/l2xUKFGnNFKm64zEYmJhOcUmEJm15w9MC1txRRF01tpKlcL3t3O16aMJgbYQkucBySV0xV2T0EsCwikOAC0Z4em6uPzSs38lkHrYBvosRRAk14EfPoEXqC5JdLxRm8ToZmGSQqt_RwHCaBE_3uLgQDVEB05Rdtkq-Xzuw30=s0-d-e1-ft was not found on this server. That’s all we know. </p>
  
  <p>It seems to be a problem that Google's proxy cannot read the php script. Both the tracking.php and the GIF-example.gif files have 775 rights and are accesible publicly.</p>
  
  <p>On Hotmail this does work so it really seems to be a problem with the Google Proxies.</p>
  
  <p>Does anybody know how to let the Google Proxies access this Tracking pixel? </p>