Interpreting Function Notation, Graphs, and Context

Vending machines are an example of how functions can be used in real life. A vending machine equation is modeled by the equation $$P=M+B$$ Where $$\begin{align} P&=\text{product received from machine}\\ M&=\text{money put into machine}\\ B&=\text{button pressed to get specific food from machine} \end{align}$$

How would this be written in function notation?