php连接MongoDB总是失败，为什么？

Question

用db.getMongo();得到的是：127.0.0.1

浏览器中访问：http://127.0.0.1:27017/
得到的是：It looks like you are trying to access MongoDB over HTTP on the native driver port.

但执行下面的代码：
<?php
// 连接到mongodb
$m = new MongoClient();
echo "Connection to database successfully";
// 选择一个数据库
$db = $m->mydb;
echo "Database mydb selected";
?>

结果是：
Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator at [e-mail] to inform them of the time this error occurred, and the actions you performed just before this error.

More information about this error may be available in the server error log.

为什么会这样？