get_file_contents('php:input') always returns null

get_file_contents('php:input') always returns null

I'm building a PHP RESTful API, following this tutorial. The following
function, which should return the data sent with the request when the
'put' method is used, returns null every time:
file_get_contents('php:input').
I've even downloaded and tested the full code example in the tutorial and
it still returns null.
I'm using cURL and the following command in order to test the 'put' method:
curl -i -X PUT -d '{"address":"Sunset Boulevard"}'
http://localhost/clients/ryan.
I've wasted days on this and still haven't gotten it to read the json
data. What am I doing wrong?