The documentation referred to is: http://us3.php.net/manual/en/wrappers.php.php

Yes it only works for streams. You cannot store data in php://memory, and later use it in another stream:

This will not work, because after file_put_contents() the stream will be closed and the data in php://memory will be lost:

file_put_contents('php://memory', 'PHP');
echo file_get_contents('php://memory');

Thta's what I figured.  Could someone add that to the docs?

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.