I Need A PHP Forum Bulletin Board Script Similar To Stack Overflow

I currently have an existing website with an existing member, registration, and user database. I would like to integrate a multi category help forum / bulletin board, something similar to that of the [login to view URL] layout but does not have to be exact. All open source scripts I have found have heavily integrated custom registrations and I don't want my users to have to register twice nor do I need the extended features. I need someone to throw something basic together from either scratch or modify an existing project to function with my current user database. - UI needs to be functional but does not have to be pretty, I will customize it to my site. (Responsive design is preferred but not a must if it keeps cost down.) - Needs to be in PHP, MySQL, HTML, CSS and meet current coding standards. - Needs to be able to post basic viewable HTML, HTML scripts, and functional URLs. (Script does not need to be syntax.) - Does not need to post javascript and other script language. - Needs basic securities associated with posting to a database, a basic hack / spam filter. - Needs basic email notification of response until question is deemed answered and a basic question / answer rating system. - Needs to use an avatar name but does not need a user pic. - Needs a basic question search system. (A search prior to question posting is also ideal to help with duplicate questions.) - Does not need a category hierarchy if it incorporates a keyword tag system. (Either or both is fine.) - Does not need text element wysiwyg for question / comment posting. (Color, font size, etc.) - Does not need other bells and whistles such as user repetition, warning systems, private messaging and so on. - Does not need admin hierarchy. I can design a back end based on a working model. (Once again if it helps keep cost down.) - SEO standards and a design that incorporates the user question in an id faux permalink is a plus. (Similar to stackoverflow.) I basically want a question and answer flow system similar to [login to view URL] that allows my existing site members to post a question and allow other existing members to to answer and add to the question. It does not need to be fancy, I can add other features as needed. I am available to help with this project in anyway needed. I currently have a user database with unique user id, verified email, and etc. I am willing to add needed columns to this such an avatar name, forum activity, and so on. Navigation and user id on my site is handled through PHP sessions. I will provide more information when hired. Please let me know if you have any suggestions or a suggested budget. Thanks.