php改变图像分辨率（dpi）没有想象力

I'm creating php image by tile small image over certain width and height. In this output image dpi is 72 by default. But I need to change it to 300. Changing exif data is enough. But I cannot use Imagick for this. Anyone can help?

$size = getimagesize($mask);
//Resize to this size
$width = $size[0];
$height = $size[1];

$im = imagecreatetruecolor($width, $height);

// Set the tile
imagesettile($im, $pattern_sys);

// Make the image repeat
imagefilledrectangle($im, 0, 0, $width, $height, IMG_COLOR_TILED);       

// Output image with resized        
$mask_sys = imagecreatefrompng($mask);
//$mask_new = imagecreate($width ,$height);

imagepng($im, $print_path);

imagecopy( $im , $mask_sys , 0 , 0 , 0 , 0 , $width , $height);

imagepng($im,$design_path);

Try this

 $imageGet = file_get_contents($imagePath);
 if($imageGet){
    $imageConverted = substr_replace($imageGet, pack("cnn", 1, 300, 300), 13, 5);
    $savefile = file_put_contents($newImagePath, $imageConverted);
  }

But just note that it will for JPG images and converting an image from 72 to 300 is really a bad idea. And may I know why you need 300 DPI image for ?