I think learning PHP as a PERL developer is very easy, but the other way around might be a little more daunting. My question is why should you want to switch from PERL to PHP. This is like saying that people who speak German should switch to English, just because it's easier to learn. As far as I'm concerned you are better of speaking German in Germany. What I'm trying to say is that sometimes programs need to be written in PERL and other times in PHP or ASP for that matter. Do not switch but increase your knowledge, it's always handy to know how to speak different languages. As for all the built-in PHP functions, it's enough to know that it's possible, you don't have to remember each one of these functions! I've been programming with PHP since version 3 and still sometimes I have to look up (thanks to inconsistent function names(I'll give you that)) in which order the function requires its parameters.

Furthermore I'd like to point out that PERL and PHP are by no means interchangeable, so learn both. In practice you cannot build all your applications in your favourite scripting language, simply because different customers use different platforms and environments. Planning a career move in just one scripting language limits you to a handful of jobs. In the end you'll see that scripting languages differ very little from each other. Some are better equipped to do some chores, while others are better at other stuff. In the end they all work in more or less the same way.

Let me point out that I love PERL and that I've also got a weakness for PHP, but this is basically because I'm an Open Source disciple. Let's not worry too much about the differences. It's possible to use both you know!