Free PHP Compilers - compile PHP scripts to native code, .NET or Java bytecode

Phalanger (.NET)

Phalanger compiles your PHP code into CLI bytecode (.exe or .dll) that can be run under the .NET 4.0 or Mono frameworks. Your PHP code can use any .NET object and library in addition to the standard PHP extensions.
The resulting .NET assemblies can be signed and obfuscated. It also provides project templates to allow you to develop PHP applications from within Visual Studio. It is released under the Apache licence. 

HipHop for PHP (native code)

HipHop translates your PHP code into C++ code, which is then compiled using the GNU C++ compiler (see the Free C++ Compilers page) into an executable binary. At the time I write this annotation, the compiler supports most of the features of PHP 5.3 (except things like eval(), of course). It runs and compiles code for 64 bit versions of Linux and FreeBSD. Since the program is distributed in source code form, you'll have to compile it yourself. It is released under the PHP Licence. 

Project Zero (Java)

Project Zero includes a compiler and runtime that can compile your PHP code into Java bytecode and execute it. Note that Project Zero is actually more than just a PHP compiler/runtime; it's a development environment that lets you develop web applications using either PHP or Groovy (another scripting language). It is available for Windows, Mac OS X and Linux. You will need to have the Java Development Kit installed. 

Roadsend PHP (native code)

The Roadsend PHP compiler produces native binaries (executables) for Windows and Linux of your PHP scripts. Your scripts are not restricted to being console (command line) programs: they can be built with an embedded web server, allowing them to work the way they do on a website, albeit on your end-user's system. The compiler is released under the GNU GPL and the runtime under the GNU LGPL. Unfortunately, the program is no longer being actively developed.