how can i change the dpi of an image with the imagick extension

[eyestorm]

I need to change all uploaded files to 72 dpi.I'm using the php imagick extension.heres what i've tried (the image i'm using is 300dpi):\[code\]$image = new Imagick();$image->setResolution(72,72) ;$image->readImage($img);$image->resampleImage (72,72,imagick::FILTER_UNDEFINED,1);$image->writeImage($target)\[/code\]this doesn't seem to anything.the image is uploading, but stays at 300dpi