I believe it is called Complex (curly) syntax. See here, about 1/3 of the way down the page: http://us3.php.net/types.string
This isn't called complex because the syntax is complex, but because you can include complex expressions this way.
In fact, you can include any value that is in the namespace in strings with this syntax. You simply write the expression the same way as you would outside the string, and then include it in { and }. Since you can't escape '{', this syntax will only be recognised when the $ is immediately following the {. (Use "{\$" or "{$" to get a literal "{$"). Some examples to make it clear:
but is hard to say without seeing more of the code...