Zend Optimizer (now Zend Guard Loader) was created to decrypt PHP files encrypted by Zend Studio/Zend Guard. It can also function as an opcode cache for PHP
Zend Optimizer/Zend Guard Loader is the server-side client for Zend's proprietary encryption of PHP code. Normally, PHP code is stored on the server in plain text. The compiler then interprets this into machine code(opcode) and that machine code is executed. Zend Guard transforms the plain text by using obfuscation techniques and then pre-compiles the PHP, thus bypassing the need to compile it first and securing the code from being reverse engineered easily.
Zend Optimizer became better known for its ability to pre-compile normal PHP files as well, and thus act as an opcode cache. It has since been surpassed in this capacity by Zend opcache, which comes built into PHP 5.5 and later
Zend Optimizer and Zend Guard Loader must be compiled into PHP to use them.
