{ } mark in echo “{$e->getMessage()}”

[quieteSmeta]

I read about this Article I wondering about this part: \[code\]try { //First try getting our user numero uno $user = new User(1); //Then, let's try to get the exception $user2 = new User('not numeric');} catch( Exception $e ) { echo "Donkey Kong has caught an exception: {$e->getMessage()}";}\[/code\]why \[code\]{$e->getMessage()}\[/code\] must be covered in bracket?
Is there any link explanation on php manual about this?i know if we don't put bracket it will show error, but want i want is to get the explanation why bracket is needed.\[quote\] Notice: Undefined property: Exception::$getMessage\[/quote\]