This section holds the most general questions about PHP: what it is and what it does.
From the manual:
PHP is an HTML-embedded scripting language. Much of its syntax is borrowed from C, Java and Perl with a couple of unique PHP-specific features thrown in. The goal of the language is to allow web developers to write dynamically generated pages quickly.
A nice introduction to PHP by Stig Sĉther Bakken can be found here on the Zend website.
PHP/FI 2.0 is an early and no longer supported version of PHP. PHP 3 is the successor to PHP/FI 2.0 and is a lot nicer. PHP 4 is the latest generation of PHP, which uses the Zend engine under the hood.
Yes. See the INSTALL file that is included in the PHP 4 source distribution.
There are a couple of articles written on this by the authors of PHP4. Here's a list of some of the more important new features:
Extended API module.
Generalized build process under UNIX
Generic web server interface that also supports multi-threaded web servers
Improved syntax highlighter
Native HTTP session support
Output buffering support
More powerful configuration system
Reference counting