An interesting presentation on Parrot's Internal Implementation (pdf) by Dan Sugalski. Don't be fooled by the sub-title: Implementing an Interpreter. Parrot can be run in Just-In-Time compiler mode - basically compiled into fast machine language, just like C or C++.

That's why the prospect of PHP6 running on Parrot is so attractive. A x10 improvement in performance for CPU intensive algorithms (such as MD5 or crc32 implemented in PHP instead of C) would not be surprising.

PS: Does anyone know the current status of the experimental project to run PHP on Parrot?

Update: Sterling Hughes gives feedback on the state of PHP on Parrot.