pixel.php

<?php
/* 
  Open tracker for email with external CSS pixel option. 
  https://freshinbox.com/blog/how-to-build-your-own-email-open-tracking-pixel/

  Parameters:
  - p:  A unique value to track.
  - ext_css (optional): Generate CSS that loads an "external CSS pixel". 
  - css_pixel: Auto generated by ext_css option that records an external CSS pixel open.

  Logging notes:
  - Request contains only "p", it will be logged as "Regular pixel fetched". 
  - Request contains "ext_css", it will be logged as "External CSS fetched". 
  - Request contains "css_pixel", it will be logged as "CSS pixel fetched". 

  To use regular pixel:
  <img src="https://myserver.com/path/to/pixel.php?p=mypixel" width=1>

  To use External CSS pixel (only for email clients that support external CSS):
  <link rel="stylesheet" href="https://myserver.com/path/to/pixel.php?p=mypixel&ext_css=1">
*/

date_default_timezone_set('America/Los_Angeles');

// You may need to pre-create pixel.log and set writable permissions
$LOG_FILE = "pixel.log";
$CSS_DEBUG = 0; // Setting to 1 will show "style sheet loaded" when using ext_css=1

/* 
  Get the parameters from the url
  ie. https://myserver.com/path/to/pixel.php?p=mypixel
*/
$p = @$_REQUEST['p'];
$ext_css = @$_REQUEST['ext_css']; // generate the CSS for an external CSS pixel (optional)
$css_pixel = @$_REQUEST['css_pixel']; // log an open by the css pixel

if(!$p){
  log_request("Test");
  die("The pixel is live!");

} elseif($css_pixel){ // preceed $ext_css since $ext_css will eval to true as well
  log_request("CSS pixel fetched");
  send_image_pixel();
  die();

} elseif($ext_css){
  log_request("External CSS fetched");
  send_css_pixel_stylesheet();
  die();

} else {
  log_request("Regular pixel fetched");
  send_image_pixel();
  die();
}
 

// Log the request
function log_request($note){
  global $LOG_FILE; 

  $p = @$_REQUEST['p'] ?: 'not_set';
  $date = new DateTime();
  $dateval =  $date->format('Y-m-d H:i:s');
  $ip = @$_SERVER['REMOTE_ADDR']; // Remote IP address
  $agent = @$_SERVER['HTTP_USER_AGENT']; // User Agent
  $referer = @$_SERVER['HTTP_REFERER']; // Referer

  $list = [$dateval, $p, $note, $ip, $agent, $referer];

  $file = fopen($LOG_FILE,"a");
  fputcsv($file,$list);
  fclose($file);
}

function send_image_pixel(){
   // https://stackoverflow.com/questions/3203354/php-script-to-render-a-single-transparent-pixel-png-or-gif/3203394
   header('Content-type: image/gif');
   header('Cache-Control: no-cache, no-store, must-revalidate');
   echo (hex2bin('47494638396101000100900000ff000000000021f90405100000002c00000000010001000002020401003b'));
}

function send_css_pixel_stylesheet(){
  global $CSS_DEBUG;

  $pixel_url = "//{$_SERVER['HTTP_HOST']}{$_SERVER['REQUEST_URI']}&css_pixel=1";

  $response = "
    body {
      background-image:url('{$pixel_url}') !important;
    }
  ";
  if ($CSS_DEBUG) {
    $response .= "
      /* for debugging in email */
      body::after{content:'style sheet retrieved';background-color:yellow;}";
  }

  header('Content-type: text/css');
  header('Cache-Control: no-cache, no-store, must-revalidate');
  echo $response;
}

?>