Just to clarify, HTML is a display/mark-up language, technically not a progamming lang
PHP is a programming lang that can be (and usually is) used to display/control interactive webpages ie can utilise HTML for the display part.
Pure HTML web pages (files) are known as static pages beacuse it's just a display lang.
PHP (or Perl or Python) can be used to generate dynamic pages becuase the HTML is often generated on the fly and you can interact with them.
You can write HTML & PHP files in any text editor eg vi, kate, gedit etc or if you want a GUI type editor for HTML, see eg BlueFish.
Here's the PHP manual: http://www.php.net/manual/en/