In addition to the functions from the standard extensions described in Appendix A, a number of optional extensions provide PHP with additional functionality. Generally, these optional extensions are interfaces to third-party code libraries. To use these functions, you need to install the libraries they depend on and recompile PHP with the appropriate compile-time directives.
This chapter is intended as a complete tour of the extensions provided with the PHP distribution, but not as a definitive reference to the functions provided by those extensions. Additional documentation for these extensions is available from the PHP web site http://www.php.net.