I have searched around for this for you, and have found the same results - At this time, there does not appear to be any existing, freely-available PHP classes to handle Craigslist Bulk Uploads.
I have also found that others, searching for the same, have resorted to posting jobs on freelancing boards like Freelancer.com to engage developers to create one for them.
Looks like you will have to write this code yourself, or pay someone to do it for you. Sorry.
APPENDUM:
Referring to the Craigslist Bulk Posting Interface help page, there is an example Perl script included at the end of the page, along with examples of the XML/RSS formats required for bulk-posting.
The simplest way to achieve what you want would be to have a PHP script create the RSS/XML File(s) and then trigger the Perl Script to perform the Upload and record the results into a second file (or straight back to the script).