PHP was originally designed to help spice up someone's home page, and stood for "Personal Home Page". As it grew, and gained more and more features, it renamed itself to the recursive "PHP: Hypertext Preprocessor".
 
PHP is not like most other languages - it is embedded within HTML, and separated from normal HTML with <?php (or simply <?) and ?> tags. This means it is simple to include a bit of PHP in the middle of a page, to display a variable or run a whole shopping cart.
 
A RISC OS PHP port was done by Alex Waugh, originally just as an executable like the Perl port, but more recently it has been built in to WebJames, the web server Alex Waugh maintains. This means a new copy of PHP does not have to be started for every PHP page requested, siginificantly speeding up PHP execution. For example, I now find it possible to browse my website (which is entirely written in PHP) either online or offline using WebJames with PHP built in, with no real noticeable loss of speed.

Links

• php.net, home of PHP
• RISC OS port of PHP