PHP is what's called a server-side scripting language.
What this means is that the website developer has written his site in the programming language PHP, so he can take advantage of programming constructs to generate dynamic page content, do calculations, interact with databases etc. Then when you request a page from his site, the web server has processed the PHP script and sent you the output from it (usually a HTML page).
Because it's been processed before it's sent to you, what you have is the HTML (and possibly CSS and JavaScript) output from the PHP script, not the PHP code itself, which is impossible for you to obtain without asking the site developer to email it to you.
Which is good from the site developer's perspective, because PHP files often reveal information about the web server, its folder structure, contain database passwords etc which would be useful to potential hackers.
If you don't care about the dynamic nature behind the pages and only need the output, then that's not a problem, you can edit the files in Frontpage or Dreamweaver if you want. You could even use a real website development tool. This all assumes, of course, that you don't mind the moral and legal ramifications of stealing somebody else's work.
On the other hand, if I've made incorrect accusations, and you actually do have the original PHP source files because you are taking over development of the website for somebody else, then I apologise.
The correct accusation to make in this case would be that you have no business trying to develop a website when you clearly have no idea what you're doing (Frontpage says it all really), and that you've grossly misrepresented your abilities to some poor sap.