The opcodes caches precompile source code so that there is not any more but to execute it at the time of the calls of the users. During the execution of a script, PHP 4 proceeds as follows: it loads the whole script and it parses it. it converts the instructions into opcodes (kind of intermediate language between script and executable). It is the compilation phase. it executes the instructions, and sends the result to the client. When the same script is called several times, these operations are renewed. This is an obvious waste of time. The solutions of opcodes caching proposes to cache the opcodes, in order to keep a trace of it. This allows PHP4 to execute them directly without carrying out the steps of reading, analysis, and compilation: Several tools coexist in this category : APC After Burner PHPA Zend Accelerator We can notice that these tools are an extension of module PHP. Their startup requires the restarting of the server and the possibility of editing the file 'php.ini' (configuration file of module PHP). APC (Alternative PHP Cache) is developed by 'Community Connect' and After Burner by 'Bware Technologies'. These two tools are free like their sources. PHPA (PHP Accelerator) is developed by Nick Lindridge, and is similar to the two preceding tools: it is free, but the sources are not available. Zend Accelerator (in the past named Zend Cache ) is developed by the team which has rewritten the PHP 4.0 engine. It is, logically, the most powerful product, but also the only one being paying. The following part presents the optimization achieved on Wysistat.
