Facebook API发布到特定页面

I'm attempting to integrate my Web App with Facebook in order to take certain messages posted to my web app and then post it on a specific, predetermined wall.

For example, when my message is posted and sent through my web app, my web app will grab a predetermined Facebook page (Using PHP) and post to that wall, regardless of what user may be signed in during that session.

I've done some browsing through the documentation of the Facebook PHP SDK, and implemented the basic like and share buttons, but I haven't found any way to maintain a permanent access token to a singular account, I've only found how to post to whichever user currently has the session.

If anyone with more experience with the API could guide me in the right direction, or tell me if this is even possible with a singular user and wall, I'd be greatly appreciative.

Thanks