Transforming Data with the Magic of Parameters If you haven’t read my introduction to user-defined functions article, make sure to do that before continuing on. If you understand what functions are, keep going. Let’s revisit the automotive plant that we looked at in the previous article. Think about an automotive plant: You bring the materials to the plant (you are here). You feed the plant with those materials (and here). The plant takes the materials and somehow produces a car. The automotive plant is the function that can be invoked to produce something. The plant() function that was created before had a local variable $type_of_plant that stored the