php imgcropauto not cropping image

I'm having trouble with php function imgcropauto.

I am trying to write a web app that will allow documents to be uploaded from various sites to a central server.

My aim is to autocrop the sides of the document out so that the image received from the users mobile device is similar quality to a scan. (similar to apps like office lens etc).

My php is below, but all it seems to be doing is rotating the image 90 degrees counter-clockwise and slightly dulls the colour.

(I have pulled out come unnecessary bits of the code and just included what is important).

if(!empty($_FILES['uploaded_file'])){
    $stamp = date("Ymdhis");
    $path = "../uploads/";
    $file = $_FILES["uploaded_file"];
    $temp = explode(".", $file["name"]);
    $newfilename = $stamp . '.' . end($temp);

    if(move_uploaded_file($file["tmp_name"], $path . $newfilename)) {
    }else{
        echo "There was an error uploading the file, please try again!";
    }
    $img = imagecreatefromjpeg($path . $newfilename);
    $cropped = imagecropauto($img, IMG_CROP_SIDES);
    if ($cropped !== false) {
        imagejpeg($cropped, $path . $newfilename);
    }
}
    else {echo "<h1>no file to upload</h1>";
}