PHP is build in C language so if you want to build extensions for PHP then you need to know C. Now, with a framework available to act as middleman, you can build PHP extensions in PHP.
Zephir is one such framework written to write PHP extension. It is in beta stage. It offers you to write your applications as low level also to protect code.
Zephir is being developed by Phalcon team to help build extension.
Phalcon is a web framework implemented as a C extension offering high performance and lower resource consumption.

Goals
The main goals of Zephir’s are:
Static/Dynamic typingAutomatic memory managementStrict and explicit behavior over flexible and implicit behaviorProduce object-oriented libraries that can be used in PHPHide unnecessary complexity to developersMake the code more debugableProduce exceptions in runtime instead of notices/warnings when possibleDetect potentially slow or unrechable code and notify the developer about itProduce faster and better code that can be compiled by most important C-compilers: (gcc, vc++, clang)Provide a very low learning curve without creating a new PHP/C implementationMain Phalcon’s goals using Zephir:
Reduce the development timeMake the code less prone to coding errorsAllow more members of the community to get involved inAllow most Phalcon’s users to read and understand how a functionality is implementedAllow developers to take more advantage of the framework fully understanding how it works by reading its codeIntroduce potential refactoring and optimizations without affect the stabilityEasily adapt the code to new PHP versionsAllow contributors to implement additional componentsWhat Zephir will not be:
The next kick-ass language for the webA replacement for PHP or C (or any other language)Be the most elegant and coherent language availableCover every possible feature (current or future) provided by PHP or CImplement every feature working exactly as in PHP or CProvide awesome expresiveness or support every possible programming paradigmMake everyone happySource:
Zephir – Build PHP Extensions Without Knowing C
Phalcon 2.0 - The future