Which of the three is generally faster? I know there are some specific cases where one performs much better than it does elsewhere but I am speaking of the common situation.
Languages are not implementations. Languages may be prone to be slow or fast when implemented, but they don't have such properties - implementations do.
Sun's Java JVM implementation is likely faster than Python's or PHP's default ones. Java compiles to bytecode which the JVM JIT's.
Python is interpreted by default, altough most third-party implementations outperform guido's by being... compilers!
It has the potential to be faster, check out the third party ones (unladen-swallow, cl-python, ...).
PHP is badly designed and has an interpreted implementation. It's probably the slowest. There is the Zend accelerator(and many third party bytecode caching plugins) which allows executing bytecode directly, but it's still SLOW. Bad language design and stupid programmers sometimes leads to people eval'ing and not caching the bytecode anyway, which defaults you to PHP's slow self.
In general, languages which only have interpreters tend to be toy languages or badly designed, or both. I'm surprised PHP and Python are so popular.
Why are you asking? I hope you're not that web design guy that made that other silly thread! If you were, I wouldn't recommend either language for what you want to do, but any of them would work. Python probably has better frameworks. Keep in mind that even if Python and PHP are slow, most web applications' speed usually hardly matters - people are willing to put up with a bit of latency. If I really wanted speed in a web framework, I'd go with one of those great Common Lisp web frameworks, and an implementation like SBCL which compiles to native code, or if that wasn't high-performance enough, I'd just write it in C.