I could not find any information on changing the DPI
information on a JPG file using the GD lib. Since changing
this does not resize or scale the actual image, it is only a
header-setting.
The following snipplet will save your $image to $file and
set the DPI to 150.
<?php
imagejpeg($image, $file, 75);
// Change DPI
$dpi_x = 150;
$dpi_y = 150;
// Read the file
$size = filesize($file);
$image = file_get_contents($file);
// Update DPI information in the JPG header
$image[13] = chr(1);
$image[14] = chr(floor($dpi_x/255));
$image[15] = chr( $dpi_x%255);
$image[16] = chr(floor($dpi_y/255));
$image[17] = chr( $dpi_y%255);
// Write the new JPG
$f = fopen($file, 'w');
fwrite($f, $msg, $size);
fclose($f);
?>
P.s. not fully tested (yet) but it works for my images ...
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