Using PHP is very easy. Let's say you want a footer to appear at the bottom of every page. So this might be typed into something like Notepad and saved as 'footer.html'. (Note: I prefer to give my include files proper extensions, not '.inc', as then I know what type they are (HTML or PHP or Text) plus any editor can open them. I also like to put them in a special folder to avoid confusion with other files, but that's just my way of doing this.)
Here's the text you might use for the footer:
<p class="footer">(C) Me 2004! No nicking this page, OK?</p>
Then all you have to do is this. Make sure your web host is running PHP first of course, or it won't work! Any files you want to be processed by PHP simply need the '.php' extension. So just save an existing HTML page and rename it.
Next, another great thing about PHP is that it can go anywhere on the page - even above the first line of your HTML!
To start some PHP code in your page, use the opening tag
[b]<?php[/b]and to end it use
[b]?>[/b]. Everything inside those tags is sent to the PHP parser.
Another great thing (there are so many!) is that you can also put in bits of HTML between these tags, only they have to be echoed. So here is what you might do to add the footer to your pages:
<p>Here is the main text on the page. Now comes the footer...</p>
echo '<p>Some more HTML here just for this page only!</p>';
If you just add the footer code on each page, then when you come to change it, all you have to do is edit the file it points to, and all your pages will be updated at once!
I've taken to making my header, footer and menu all includes.