PHP: JPEG/JPG DPI function

While working with a lot of JPEG images lately I needed to be able to set the DPI with PHP and I was having trouble finding a way to do that with the native PHP or GD functions. The stuff I did find seemed to be overly complicated or just plain wrong.

So I wrote my own DPI function for PHP that rather than doing pointless resampling simply adjusts the file header.
My needs didn't require me to do it "right", but it will handle from 1 to 255 DPI.

/*
   @$jpg -- Path to a jpg file
   @$dpi -- DPI to use (1-255)
*/
function set_dpi($jpg, $dpi = 163)
{
 $fr = fopen($jpg, 'rb');
 $fw = fopen("$jpg.temp", 'wb');
 
 stream_set_write_buffer($fw, 0);
 
 fwrite($fw, fread($fr, 13) . chr(1) . chr(0) . chr($dpi) . chr(0) . chr($dpi));

 fseek($fr, 18);
 stream_copy_to_stream($fr, $fw);

 fclose($fr);
 fclose($fw);
 
 unlink($jpg);
 rename("$jpg.temp", $jpg);
}