I’m not really sure why, but I can guess. Do you have a file in bb-includes called compat.php? /bb-includes/compat.php
In that file, line 10 says “if this function does not exist, create it.” That PHP function is available in newer PHP versions only (greater than or equal to 5.1.2). So, for compatibility, if the function is not available on your server, bbPress creates it in compat.php.
Do you have that file, and does line 10 look like this?
if ( ! function_exists('hash_hmac') ):
What bbPress version did you install, if it’s not the latest 0.9.0.2 release? Are you certain all the files were uploaded properly?
There is a slight second possibility.
Their PHP may actually have that function, but it’s disabled.
Since the bbPress fallback function only checks if it exists (and it does) it never gets defined. But it’s disabled.
However your questions need answering too, what version, properly uploaded, etc.
