As a certified zend php developer (ZCE) I'd opt for PHP, but there's more to it. Today PHP is reaching it's maturity supporting many features that are already available for ASP (.NET) and JSP (Java). In my opinion a future-proof technology and a perfect glue between the Java and .NET worlds. And it's perfect for today's Web 2.0 applications (facebook, flickr, yahoo, ...)
But it all comes down to a few simple facts:
- what technology do you master in house (.NET, Java or PHP) ?
- do you already have an infrastructure available ?
- where lies your concern when it comes down to support ?