在wordpress中创建ajax调用。 我需要包括什么才能访问wordpress功能

Question

I am performing an jquery ajax request inside my wordpress. This calls an internal php script. This php script needs to be able to access certain wordpress features like... functions.php which is simple for me to include. What i cant do is access info like the current wordpress user, the $wpdb object. My question is... is there some wordpress file which i can include which gives me access to all that data (and functions.php). I hope you understand what i am accessing as i am aware that was probably THE crappest explaination in the world :D