PHP (officially "PHP: Hypertext Preprocessor") is a server-side
HTML-embedded scripting language.
Simple answer, but what does that mean? An example:
Example 1-1. An introductory example
<html>
<head>
<title>Example</title>
</head>
<body>
<?php echo "Hi, I'm a PHP script!"; ?>
</body>
</html>
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Notice how this is different from a CGI script written in other
languages like Perl or C -- instead of writing a program with lots
of commands to output HTML, you write an HTML script with a some
embedded code to do something (in this case, output some
text). The PHP code is enclosed in special start and end tags
that allow you to jump into and out of PHP mode.
What distinguishes PHP from something like client-side Javascript
is that the code is executed on the server. If you were to have a
script similar to the above on your server, the client would receive
the results of running that script, with no way of determining what
the underlying code may be. You can even configure your web server
to process all your HTML files with PHP, and then there's really no
way that users can tell what you have up your sleeve.
