works by caching the compiled form of a PHP script (opcodes) in shared memory to avoid the overhead of parsing and compiling the code every time the script runs. An opcode cache, Zend Opcache, is built into PHP since version 5.5. Another example of a widely used opcode cache is the Alternative PHP Cache (APC), which is available as a PECL extension. While Zend PHP is still the most popular implementation, several other implementations have been developed. Some of these are compilers or support JIT compilation, and hence offer performance benefits over Zend PHP at the expense of lacking full PHP compatibility. Alternative implementations include the following: HHVM (HipHop Virtual Machine) – developed at Facebook and available as open source, it converts PHP code into a high-level bytecode (commonly known as an intermediate language), which is then translated into x86-64 machine code dynamically at runtime by a just-in-time (JIT) compiler, resulting in up to 6× performance improvements. However, since version 7.2 Zend has outperformed HHVM, and HHVM 3.24 is the last version to officially support PHP. HipHop – developed at Facebook and available as open source, it transforms the PHP scripts into C++ code and then compiles the resulting code, reducing the server load up to 50%. In early 2013, Facebook deprecated it in favor of HHVM due to multiple reasons, including deployment difficulties and lack of support for the whole PHP language, including the create_function() and eval() constructs. Parrot – a virtual machine designed to run dynamic languages efficiently; the cross-translator Pipp transforms the PHP source code into the Parrot intermediate representation, which is then translated into the Parrot's bytecode and executed by the virtual machine. PeachPie – a second-generation compiler to .NET Common Intermediate Language (CIL) bytecode, built on the Roslyn platform; successor of Phalanger, sharing several architectural components Phalanger – compiles PHP into .Net Common Intermediate Language bytecode; predecessor of PeachPie Quercus – compiles PHP into Java bytecode Licensing PHP is free software released under the PHP License, which stipulates that: This restriction on use of "PHP" makes the PHP License incompatible with the General Public License (GPL), while the Zend License is incompatible due to an advertising clause similar to that of the original BSD license. Development and community PHP includes various free and open-source libraries in its source distribution, or uses them in resulting PHP binary builds. PHP is fundamentally an Internet-aware system with built-in