json中如何将key中的引号去掉

function my_json_decode($str) {        //       
    $str = preg_replace('/"(w+)"(s*:s*)/is', '$1$2', $str);   //去掉key的双引号        
    return $str;    
}    
$str = '{"test":[{"testName":"哈哈","Url":"http://www.test.com"}]}';    
$str = my_json_decode($str);    
echo $str;