PHP is a server-side scripting language. I can't imagine why you would need it in place of a slideshow. I think what you're looking for is JavaScript or CSS.

Sublime Text 3 is a sort of enhanced notepad with syntax highlighting and code completion. It may not be what you're looking for since you want a visual editor but it could be something to look into.

I've used CoffeeCup HTML editor in the past, but I can't really vouch for any of these because markup is so incredibly easy that I can't imagine needing an editor for it. You might as well use MS Word for the WYSIWYG feel that I think you're looking for.