As Wez Furlong puts it on his weblog:

Marco aims to publish a number of case studies on the adoption of PHP in small, medium, and large businesses (or enterprise, if you will) with the intention of creating a kind of marketing "war chest". The idea is to form a kind of business PHP hub that will help other businesses to make a more informed decision when it comes to adopting PHP.

Marco is looking for people to write case studies in marketing-speak to use there; for more information, read Marco's blog entry.

The basic idea here is to create a repository of information to aid in the promotion of PHP, and provide a good marketing perspective on how and why PHP has been adopted in the past. Also: do remember that these are marketing documents, not technical ones. As such, you must be ready to write from a marketing and business point of view, rather than a technical one.