Issue 17 of 30 Days of Hack (HHVM)
A tuple is very similar to a PHP array and a shape. The key feature of tuples is that they are immutable. You may not change the length or the types of the elements in a tuple. The values within the tuple are mutable.
You should use a tuple when you function needs to return more then one value. You do not predefine the types within the tuple but the return annotation of the function has changed. If your function returns a tuple with an integer and a string, then the return type annotation should look like this: (int, string)
Read more at the developer blog.