PHP, originally derived from Personal Home Page Tools, now stands for PHP: Hypertext Preprocessor, which the PHP FAQ describes as a "recursive acronym." PHP is an alternative to Microsoft's Active Server Page (ASP) technology. As with ASP, the PHP script is embedded within a Web page along with its HTML. Before the page is sent to a user that has requested it, the Web server calls PHP to interpret and perform the operations called for in the PHP script. An HTML page that includes a PHP script is typically given a file name suffix of ".php" ".php3," or ".phtml". Like ASP, PHP can be thought of as "dynamic HTML pages," since content will vary based on the results of interpreting the script. Since it's a preprocessor, it runs on the remote web server and processes the webpages before they are sent to the browser. This makes it a so-called server-side scripting language. The fact that it runs on the server has several benefits, and some drawbacks. Let's take the benefits first:

On the server you can have access to things like a database. This means that you can make a script that sorts through large amounts of data, without the client having to download them first.
It's only the output from the script that is sent to the client, not the script itself. That means that you can make the script invisible from the end-user. That makes PHP-scripts browser-neutral; they don't depend on some capability of the browser. It also means that you don't have to worry that someone else can steal your carefully crafted script. It's not like when you make a JavaScript --- everybody is able to read it in the source of the webpage. It has to be this way with client-side scripting, or else the client would be unable to get the source of the script, and therefore unable to do any scripting.
You can make your own programs for use in your scripts. You could implement part of the script in C, and then call the program from your script to make it run faster. PHP is a parsed language, meaning that there are no compiled binaries. Every time someone requests a page with PHP-code, the parser looks through the page and executes any PHP-statements it might find. Fortunately this is a very fast process, but you might want to speed things up if you have a very complicated script.

When you make a C-program, you compile the source and then run the resulting binary. This makes PHP slower than an equivalent C-program.

As I said, there are also some drawbacks:

By executing everything on the server, you put more strain on it. With many concurrent requests, and large complex scripts, the server might not be able to handle it. But this isn't a real concern because the parser in PHP is so quick. And if your server still can't cope with the number of visitors, then you should be able to generate some revenue from banners on your site, and then pay for a bigger server!
The pages can't do anything themselves --- you need the server to do the magic. This means that the pages will lose some of their functionality if your visitors decide to save them to their computer.

You could of course still put some JavaScript in your pages. This is a very powerful combination between server- and client-side scripting. You could use PHP to fetch some values from a database, and then setup your variables in the JavaScript with these values.

I find that PHP offers a site a more flexibility and is more dynamic than HTML can offer on its own.

Some Background Information

PHP is actually a rather simple language, despite its great powers. It's a very young language, so the developers have had the chance to learn from previous language's mistakes and implement their strengths. Much of the syntax is borrowed from C. This is reflected in the different conditional statements, the loop-structures, the Boolean operators, and the assignment of variables. Since C is probably the most common programming language today, this should make PHP easy to pick up. Even if you don't have any previous experience with C you should be able to learn it quickly. One can just apply the techniques learned from PHP when one programs in C. Of course this is only true as long as you only write simple programs in C, since C is a "real" programming language, suitable for writing operating-systems in or the like.

But another thing that makes PHP easy to learn is it's relaxed way of dealing with the types of variables. Its very simple: you don't have to think of the types of variables at all! If you assign a number to a variable, then it just works. When you later try to output the variable to the browser it also just works. PHP takes care of converting the variable from an integer-type to a string-type, on the fly and automatically. To make matters even simpler, you don't even declare your variables --- you just assign a value to them, and then they are ready.

Because PHP is meant to be used with webpages it has a lot of functions to deal with text. Generally you can say that PHP is specially designed to deal with webpages, and doing so quickly and efficiently. Because of that most of the built-in functions are simple and straightforward to use.

Being web-oriented, PHP also contains all the functions you'll need to do things on the Internet. There are functions for connecting to remote webservers, checking mail via POP3 or IMAP, or url encoding strings to protect special characters.

The World Wide Wonderland (as they say) will contain a huge amount of PHP related informatiom (see the Resources section) in case you get stuck! Feel free to contact me if you want any additional information or want to correct or add anything to this document.