Having difficulty changing DPI on JPEG with PHP

[markurxn]

I need a solution to automatically change the DPI of uploaded photos from 300 to 72. I've coded this bit of script to read the 14th through 18th bit of binary data of a JPEG and update the DPI. The script below seems to be writing to the jpeg, but the afterwards, the jpeg is unreadable and when I rerun the script, it still shows the image DPI hasn't changed. Anyone have experience changing DPI of an image?

<?php
 
  $filename = 'familyshot33.jpg';
 
  // open file for writing
  $f = fopen($filename, 'r+');
 
  $string = fread($f,20);
  // show dpi before 
  echo hexdec(bin2hex(substr($string, 14, 2))) . ' ' . hexdec(bin2hex(substr($string, 16, 2))) .'<br/>';
 
  // update dpi
  $image = substr_replace($string, pack("Cnn", 0x01, 72, 72), 13, 5);
 
  // show dpi after
  echo hexdec(bin2hex(substr($image, 14, 2))) . ' ' . hexdec(bin2hex(substr($image, 16, 2))) .'<br/>';
 
  // write and close
  fwrite($f, $image,  filesize($filename));
  fclose($f);
 
?>
 

[pbmods]

Heya, Markurxn. Welcome to Bytes!

You may find this thread to be of some interest.