strtr array replace issue...

Apache: 2.2.8 threaded
PHP: 5.2.4-2
mySQL: 5.051.a

Ubuntu 8.04 minimal with everything needed installed via apt-get.

Using SMF forum, I'm having an issue with the strtr command...  Here's
the code...

$ftp_file = strtr($filename, array ($_SESSION['pack_ftp']['root'] =>''));

The $_SESSION['pack_ftp']['root'] variable is an empty set: ''

Well, if the above strtr command was translated it would look like this:

strtr("/home/user/public_html/Packages/temp", array ('' =>''));

You would think that the return would be : /home/user/public_html/Packages/temp

However, it returns '' instead.

I can get around this by modifying the code to the site manually and
checking to see if $_SESSION['pack_ftp']['root'] == '' and if it does
assign it something crazy, like $_SESSION['pack_ftp']['root'] =
'%%%%%'

However, the code works on another site I have that's hosted for me
(shared hosting).

Anyone have any ideas?

I was wondering if it was related to this: http://bugs.php.net/bug.php?id=27457
But that seems like an old bug...

Thanks.