I agree that there is no point in rewriting everything in PHP. It wastes time, and time is money.
However, the two languages are similar. After knowing Java for a couple of years, PHP was not hard at all, and I could still use either my NetBeans IDE or Eclipse IDE. So, if you want your PHP developers to learn Java or your Java developers to learn PHP, this is not a problem in my opinion to worry about. It'd be very easy.
There are a lot of great features about the PHP language: typeless variables, heredoc syntax, readability, etc. I would say some cons are less information and control over errors, not as good unicode support (if you're using other languages besides English), and less object-oriented design as Java (you can have classes, but it's still a lot more like C than C++ or Java).
Here's a link I found:
http://www.c2.com/cgi-bin/wiki?PhpProsAndConsI love PHP and Java, so it's hard to say really. And like everyone has already said, with a large system, you can't beat the heavily object-oriented design of Java, while PHP shines in easiness, speed, and readability of the data going out/coming in (you can look at a PHP file and pretty much know what it will look like).