A PHP accelerators are designed to improve the performance of PHP apps. Most of them cache the compiled opcode/bytecode to avoid the overhead for compiling the source code on each request. The cached code is stored in a shared memory and executed from there. As a result, PHP accelerators minimize the amount of slow disk reads and decrease the memory copying at a runtime.

The effect on performance varies widely as it depends on different factors. The inherent execution time or the percentage or actually executed source code plays the significant role in this process.  The additional optimization steps are also important. A single code optimizer may slow down the overall performance of the website, but coupled with PHP accelerators it can boost the performance.

Alternative PHP Cache (APC)

Alternative PHP Cache or APC is a free, open source framework designed to cache the  PHP bytecode compiler output in a shared memory. The accelerator reduces parsing and disk I/O overhead, as well as a user data shared memory cache. 3x increase in a page generation speed is the maximum possible result. Thanks to numerous contributors, the framework has a mature codebase. APC works with all PHP versions. You can find the official installation help here and here.

XCache

XCache is a PHP opcode cacher for production servers under high load. The solution is tested on Linux and FreeBSD. At the same time, it is supported under Windows for both thread-safe and non-thread-safe PHP versions. The software is developed by mOo – the creators of Lighttpd. It helps to overcome some of the existing limitations. Xcache provides the full support for PHP up to 5.6. The solution is actively maintained, as a result, we can see different stable, unstable, and developer releases. The official installation guide is here.

Nusphere PhpExpress

NuSphere PhpExpress is another free solution, that works with PHP opcode cache. It loads both plain PHP and Nu-Coder encoded files into the PHP engine to save loading time and boost performance. NuSphere PhpExpress is available on Linux, Windows, Mac OS X, NetBSD, FreeBSD, and Solaris. The solution is extremely easy to install and use, at the same time, it provides a significant increase in PHP scripts execution speed. PhpExpress by NuSphere can be downloaded and distributed for free without any limitations.

Zend Opcache

Zend Opcache is a component of Zend Server. Being open source, it speeds up PHP execution by optimization and caching of opcode. Zend Opcache relies on shared memory to store precompiled script bytecode. This eliminates two major stages – reading code from the disk; and compiling code on future access. Keep in mind, that Zend Opcache doesn’t load Zend Guard encoded files. You need Zend Guard Loader in order to achieve this feature. Zend Opcache is integrated into PHP starting with PHP 5.5. The accelerator is actively maintained, and you can find more information on its GitHub page.

Windows Cache Extension for PHP

This free and open source PHP accelerator was developed for PHP under Windows. Besides PHP opcode cache, it also includes file cache, object/session cache, resolve file path cache, file change notifications, lock/unlock API’s. Such set of caches significantly improves PHP applications performance on Windows. Being actively developed, Windows Cache Extension for PHP works with x86 versions of PHP 5.2-5.5. Hit this link for help and API documentation.

eAccelerator

This is another PHP accelerator and optimizer, which improves the performance of PHP scripts by caching them in a compiled state. It helps to avoid the overhead of compiling as much as possible. eAccelerator also optimizes scripts to increase the speed of their execution. The solution typically increases the PHP code speed by 1-10 times and reduces server load. the scripts can be executed by several engines at the same time. Some additional information: appropriate Github repository and official installation help.

ionCube PHP Encoder

The availability of ionCube PHP Accelerator or PHPA on a wide variety of platforms led to its adoption from small sites to such giants as Yahoo!. There are three editions of the ionCube PHP Encoder – Basic, Pro, and Cerberus. All of them offer compiled PHP encoding and optional encryption for non-PHP files. Pro and Cerberus provides some additional features. The ionCube PHP Encoder is available for Linux, Windows, FreeBSD, OS X. All editions provide easy automation with the help of command line Encoders. The Pro and Cerberus versions also include different tools for the creation of license file.