A: 

Check this out if it's any help: http://stackoverflow.com/questions/3536920/post-on-a-friends-wall-on-behalf-of-a-user

TuomasR
there is no real answer there. Only answer sends to http://developers.facebook.com/docs/reference/rest/stream.publish , which doesn't work as I said, if the person to whom we send the notification didn't authorized the application to post on the wall (((((
DR.GEWA
It might be that you can't. The Facebook application posting rules have been tightened considerably lately, with the removal of notifications and other changes. You might be out of luck.
TuomasR
Can somebody tell me how using $facebook->api() make a wall post on user's wall???
DR.GEWA
A: 

There is no way to post on another persons wall directly. If you want to invite them to user your app you will have to use the request-form-submit fbml tag. You can find this here: http://developers.facebook.com/docs/reference/fbml/request-form-submit. After that user has allowed your app you can then post on their wall.

Nathan Totten