This post on the PHPImpact blog talks about a compiler that helps to bridge some of the gap between PHP and .NET - Phlanger.

Phalanger is a new PHP implementation introducing the PHP language into the family of compiled .NET languages. [...] Phalanger compiles PHP scripts into MSIL (Microsoft Intermediate Language) which can be executed by .NET or Mono runtime.

He mentions some of the features of the tool (including interoperability between .NET and PHP and the ability to use current PHP libraries inside the .NET application). More information can be found on the Phlanger website.

In this post from his blog, Jacob Santos asks the question "would PHP benefit from JIT compilation?"

JIT (just in time) compilation is, according to Wikipedia, a method for converting, at runtime, code from one format into another, for example bytecode into native machine code. To try to see the benefit of this, Jacob asks four questions in his post (each with their own problems):

• Q: What advantage over opcode caching is gained?
  P: A JIT library that works with PHP would have to support all of the architectures that PHP can run on!
• Q: Which JIT library is the best one?
  P: A JIT compiler is not going to achieve anything without either storing the results or caching them.
• Q: Wouldn't writing PHP extensions negate the need for JIT compilation?
• Q: When will I develop and release my implementation?