The Zend Optimizer+ is open source
The Zend Optimizer+ provides faster PHP execution through opcode caching and optimization. It improves PHP performance by storing precompiled script bytecode in the shared memory. This eliminates the stages of reading code from the disk and compiling it on future access. In addition, it applies a few bytecode optimization patterns that make code execution faster.
This version of Zend Optimizer+ is compatible with PHP 5.2.*, 5.3.*, 5.4.* and PHP-5.5 development branch. PHP 5.2 support may be removed in the future.
For source code and more information go here.