PHP4 is the latest incarnation of PHP the "PHP Hypertext Preprocessor". It's a programming language for building
dynamic, interactive web sites, originally devised by Rasmus Lerdorf way back in 1994. Since then it's been through
a great many changes, and has been adopted by web programmers all around the world. So what exactly is it?
In technical terms, PHP4 is a cross-platform, HTML-embedded, server-side web scripting language. Let's take a
moment to define these terms:
❑ Cross-platform
You can run most PHP4 code, without alteration, on computers running many different operating
systems. A PHP4 script that runs on Linux will generally run on Windows as well.
❑ HTML-embedded
PHP4 code is written in files containing a mixture of PHP instructions and HTML code.
❑ Server-side
The PHP4 programs we write are run on a server specifically, a web server.
❑ A web scripting language
We run PHP4 programs via a web browser. We access the web server on which they reside, and this
runs the program, sending any resulting output back to the browser.
This means that we're going to be writing programs that mix PHP4 code and HTML together, using the former to
control and format the latter. We'll then need to put those programs onto a web server to run them. Finally, we're
going to access them from a web browser, which displays the resulting HTML. This means our programs can be
made available for other people to access across the Web, simply by placing them on a public web server.
You're probably already familiar with HTML "HyperText Markup Language". This is the language used to write
web pages, combining plain text and special tags that tell a browser how to treat that text. We use HTML to describe
how different elements in a web page should be displayed, how pages should be linked, where to put images, and so
on.