Converting an app from (or example) ASP to PHP would pretty easy -- time consuming, perhaps, but do-able, as they're both C-syntax languages. It's like the difference between French and Italian, more similar than different.
CFML to PHP are like French and Swahili. It can be done, but you'd basically be rewriting everything from ground-up. If it's a small site or project, go for it, but if you're talking more than half a dozen good-sized pages...