PHP - Add transparent text watermark to an image to make it binary unique

transparent_text_on_image.php

<?php
  // Create image resuource, use 'imagecreate*' that you need 
  $im = imagecreatefromstring($img_string);
  imagesavealpha($im, true);
  
  $trans_colour = imagecolorallocatealpha($im, 0, 0, 0, 127); // transparent background
  imagefill($im, 0, 0, $trans_colour);
  
  $text_color = imagecolorallocatealpha($im, 0, 0, 0, 126); // almost transparent text color "not visible human eye"
  imagestring($im, 1, 0, 0,  "UNIQUE_ID", $text_color);
  
  ob_start ();
  
    imagepng ($im);
    $image_data = ob_get_contents ();
  
  ob_end_clean ();
  
  imagedestroy($im);
  $img_string = $image_data;
  
?>