The Horde project released a first PHP PEAR package representing a small subpart of the Kolab functionality within the Horde framework: Horde_Kolab_Format.
The package allows you to read and write the Kolab XML format. The XML data is converted from or to a data array within PHP. Some examples are being given on a page in the Kolab wiki.
The package is the first in a series of five packages that will be released over the next few months. The full set of packages will allow you to easily deal with data stored on a Kolab server within your own web applications.