DreamHost PHP runs PHP as CGI by default, and there is no "supported" way to run mod_php your domain. It may be possible, and even advisable, to attempt to make the necessary changes to the script in question to enable proper operation under PHP-CGI, but I understand that may be beyond the scope of what you are willing to undertake.
That said, you may be able to run php scripts on your domain with mod_php, as indicated here. That page links to more information about mod_php on Dreamhost (make sure you read the comments!), though this is not "supported" and, if it works on your server (it does on mine), it might quit working at any time.
Considering how much older software relies on mod_php, I suspect it may remain available via this method for a while yet, but I could be wrong.
I think it is better to get the thing working with PHP-CGI, but YMMV.
[color=#CC0000]Edited to provide additional info:[/color] I took a quick look over at the phpLD forums for you, and it appears that your issue has been sorted out over there. You might pop over there for additional research on getting phpLD to run properly on DreamHost using PHP-CGI. As many people host on DreamHost, it is generally likely that a popular application will have already gotten any DreamHost specific installation problems sorted, so checking the applications' support forums is always a good idea.