How do we appropriately credit the kind souls who seemingly work anonymously within the php.net CVS repository? Not sure, but here are about 8 reasons why you should write for the official PHP manual.
Among the items on his list (longer than eight reasons) are things like "You'll own a cool @php.net email address", "You'll gain experience using DocBook and the related tools to create documentation", "You'll make new friends" and "You'll help make something better, something you use. It's good to use things that get better, right?"
Check out his full post for the rest of his reasons.