如何使用exif将图像dpi更改为300

Question

I have a jpeg image.I want to change the dpi of this image to 300.

Is this possible with exif installed on webserver.

If yes ,How can I do this?

$path='/home/mydev/mymagento/media/custom_product_preview/predefined_images/simpletext.jpg';            

                    $image = file_get_contents($path);              

                    $image = substr_replace($image, pack("Cnn", 0x01, 200, 200), 13, 5);
                        //  Mage::log($image,null,'DPI.log'); 

                    header("Content-type: image/jpeg");
                    header('Content-Disposition: attachment; filename="'.basename($path).'"');
                    echo $image;

I am using above code for this.Please help?