How to generate randomized name for image file after resizing it

Question

i have a code here that generates resized images but it generates a fixed name $url = 'destination.jpg'; so here is my code instead of generate a fixed name how can i change the image name to random numbers and letters. eg $url = 'wy3vu45.jpg'

if ($_POST) {

        if ($_FILES["file"]["error"] > 0) {
                $error = $_FILES["file"]["error"];
        } 
        else if (($_FILES["file"]["type"] == "image/gif") || 
        ($_FILES["file"]["type"] == "image/jpeg") || 
        ($_FILES["file"]["type"] == "image/png") || 
        ($_FILES["file"]["type"] == "image/pjpeg")) {

                $url = 'destination .jpg';

                $filename = compress_image($_FILES["file"]["tmp_name"], $url, 50);
                $buffer = file_get_contents($url);

                /* Force download dialog... */
                header("Content-Type: application/force-download");
                header("Content-Type: application/octet-stream");
                header("Content-Type: application/download");

        /* Don't allow caching... */
                header("Cache-Control: must-revalidate, post-check=0, pre-check=0");

                /* Set data type, size and filename */
                header("Content-Type: application/octet-stream");
                header("Content-Transfer-Encoding: binary");
                header("Content-Length: " . strlen($buffer));
                header("Content-Disposition: attachment; filename=$url");

                /* Send our file... */
                echo $buffer;
        }else {
                $error = "Uploaded image should be jpg or gif or png";
        }
}