function formatbytes($file, $type)   
{   
    switch($type){   
        case "KB":   
            $filesize = filesize($file) * .0009765625; // bytes to KB   
        break;   
        case "MB":   
            $filesize = (filesize($file) * .0009765625) * .0009765625; // bytes to MB   
        break;   
        case "GB":   
            $filesize = ((filesize($file) * .0009765625) * .0009765625) * .0009765625; // bytes to GB   
        break;   
    }   
    if($filesize <= 0){   
        return $filesize = 'unknown file size';}   
    else{return round($filesize, 2).' '.$type;}   
}

// USAGE   
echo formatbytes("$_SERVER[DOCUMENT_ROOT]/images/large_picture.jpg", "MB");   
// would display the file size in MB