هناك خطر هنا.

Because they are doing different jobs.

• هناك at the start often works like the English existential there is / there are
• هنا gives the actual location: here

So هناك خطر هنا is literally something like There exists danger here.

In present-tense Arabic, the verb to be is usually not stated in simple sentences like this. Arabic commonly leaves it out.

So English There is danger here becomes simply:

• هناك خطر هنا

But in the past or future, Arabic would normally use a verb, for example:

• كان هناك خطر هنا = There was danger here

Here, هناك does not mean a literal far-away there. It is being used in a very common MSA pattern to introduce existence, like English there is.

So in this sentence:

• هناك = there is / there exists
• not really over there

In other sentences, though, هناك can still mean the normal location word there.

Because existential sentences usually introduce something indefinite:

• خطر = danger / a danger
• الخطر = the danger

So هناك خطر هنا means There is danger here, not The danger is here.

If you said:

• الخطر هنا

that would mean The danger is here, which is a different idea.

It would usually be written as:

• هُناكَ خَطَرٌ هُنا

A few useful notes:

• خطرٌ ends with ـٌ because it is indefinite and nominative in this sentence
• In normal Arabic writing, these short vowels are usually omitted, so you normally see هناك خطر هنا

A simple pronunciation guide is:

• هناك = hunāka
• خطر = khaṭar or ḫaṭar
• هنا = hunā

Key sounds:

• خ is a throaty sound, like the ch in Scottish loch or German Bach
• ط is a heavier, emphatic t
• The ā in هنا and هناك is a long vowel

Yes. That is another very common way to say it.

• هناك خطر هنا = There is danger here
• يوجد خطر هنا = There is danger here

The difference is mostly stylistic:

• هناك خطر هنا uses the existential there is pattern
• يوجد خطر هنا uses the verb يوجد = exists / is found

Both are correct in Modern Standard Arabic.

Yes, but the meaning or emphasis can change.

• هناك خطر هنا = There is danger here
  This is a very clear standard existential sentence.

• هنا خطر = There is danger here / Here is danger
  This can work, especially in context, but it is less neutral.

• الخطر هنا = The danger is here
  This is different, because الخطر is definite.

So the original order is a very natural way to express the idea.

No. خطر is a general word and can refer to:

• danger
• risk
• hazard
• threat

The exact shade of meaning depends on context. In a warning sign or safety context, danger or hazard is often the best translation.

A standard way is:

• ليس هناك خطر هنا

A shorter, sign-like version is:

• لا خطر هنا

Both mean There is no danger here, but ليس هناك خطر هنا is the fuller sentence.