Why do template engines use double curly braces?

Question

PHP

Why do template engines use double, triple or even quadruple curly braces and not just single ones?
Does anyone know the real reason?

Answer 1

In my opinion, the reason is obvious - single brackets will conflict, for example, with JS.
and double openings are hardly used anywhere. it happens that template engines conflict (for example, symfony2 and in angularJS). In this case, for example, Angular supports setting arbitrary characters for templating engines.

Answer 2

Suddenly, because single curly braces have a special meaning in many programming languages.

Answer 3

Not to be confused with language constructs.