PHP

PHP is a server-side scripting language.

Running PHP as an Apache module vs CGI vs FastCGI

Caching

Memcached can be used as a peristent, cached key-value data store.

How to install and use Memcached on Ubuntu 12.04

PHP The Right Way, Object Caching

Debugging

XDebug is an extension which provides debugging information, profiling capabilities, interactive script debugging with a debugger front-end, and code coverage analysis.

PECL and PEAR

PECL is a PHP extension repository, with extensions written in C that can be loaded into PHP to provide additional functionality.

PEAR is a PHP library repository, with libraries written in PHP that can be loaded in your application.

A package manager for PECL and PEAR extensions is available for the command line.