php realpath（）返回一个空字符串

Guided by this post Preventing Directory Traversal in PHP but allowing paths I am trying to prevent directory traversal, but I am experiencing something odd, in the first instance realpath returns a good path

        $fPath = $path.$parent.'/'.$name;
//$name (the user input) in this sample is "fff", $parent is an empty string        

        //make sure user didn't try to traverse backwards on the basepath           
        $basepath = $path;
        $realBase = realpath($basepath);
echo $realBase."<br/>";
//gives: /Users/me/Documents/www/gallery/php_sample/uploadedImages

        $userpath = $fPath;
echo $userpath."<br/>";
//gives: /Users/me/Documents/www/gallery/php_sample/uploadedImages/fff

        $realUserPath = realpath($userpath);
echo $realUserPath."<br/>";
//gives blank (an empty string).

Any idéas on why?