Also related....(params)

I am trying to create a function that will check if a variable exists, if it does leave it alone, if not create it. This is similar to <cfparam> for those who know cold fusion. Anyone know a good way to deal with this in php4? I just started learning php, im sure someone has had to deal with this for some reason, there must be a good way, I trust php, dont fail me now baby! thanks....