What is PHP?
From the preface of 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.
What does PHP stand for?
PHP stands for PHP: Hypertext Preprocessor. This confuses many people because the first word of the acronym is the acronym. This type of acronym is called a recursive acronym. For more information, the curious can visit » Free On-Line Dictionary of Computing or the » Wikipedia entry on recursive acronyms.
What is the relation between the versions?
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 5 is the current generation of PHP, which uses the » Zend engine 2 which, among other things, offers many additional OOP features.
Can I run several versions of PHP at the same time?
Yes. See the INSTALL file that is included in the PHP source distribution.
What are the differences between PHP 4 and PHP 5?
While PHP 5 was purposely designed to be as compatible as possible with previous versions, there are some significant changes. Some of these changes include:
A new OOP model based on the Zend Engine 2.0
A new extension for improved MySQL support
Built-in native support for SQLite
A new error reporting constant, E_STRICT, for run-time code suggestions
A host of new functions to simplify code authoring (and reduce the need to write your own functions for many common procedures)
For more detailed information, please view the section on Migrating from PHP 4 to PHP 5 and the section on Backwards Incompatible Changes.