particles - は/が with いる. When to use each of them

My textbook says the correct structure is

WHEREにWHOがいる

For example:

日本にかおるさんがいる

But I've seen some webpages with examples such as:

かおるさんは日本にいる

So... are they both correct? Do they have different meanings? Is one of them wrong?

Answer

Both are correct but their meaning isn't exactly the same.

An easy way to understand it is to think of them as answers to different kind of question :

庭に犬がいる。

(There is) a dog in the garden.

Can be the answer to :

庭に何がいる？

What's in the garden?

Whereas :

犬は庭にいる。

The dog is in the garden.

Can be the answer to :

犬はどこにいる？

Where is the dog?

In the former example, the word precedeing が is the new information (which in this case can be translated with the indefinite article "a"), the hearer and the speaker both know which garden they are talking about.

EDIT : This distinction between known and unknown information is probably 100% true when　に is followed by は (庭には...) but 庭に犬がいる can also mean "there is a dog in a garden" with both informations being new to the hearer.

In the latter example, the word preceding は is the known information (translated with the definite article "the" here, they know which dog they are talking about), and the word preceding に is the new one.

So in your case :

（誰が日本にいる？）日本にかおるさんがいる。　

(Who is in Japan?) (It is) Kaoru (that) is in Japan.

VS

（かおるさんはどこにいる？）かおるさんは日本にいる。

(Where is Kaoru?) Kaoru is in Japan.

You can find a similar construction with だ :

僕は学生だ。 (What are you?) I'm a student.

VS

僕が学生だ。 (Who is the student?) I'm the student.