If you had searched this forum before posting a question, you would have found the answer below, which I've copied for your convenience:
I'm giving you an example, based on how I've setup my guestbook, but the same technique can be used for any kind of 'small' application written in a script language like PHP.
In my case, I've setup the guestbook in a directory of it's own on the website. The directory is called {domain-name.com}/guestbook/.
In a different part of the site, I have a web page, called gastenboek.htm (it's a site in Dutch, gastenboek means guestbook).
Here is a small piece of code from this gastenboek.htm:
As you can see, I've given the Iframe a name (Guestbook), and some visual settings such as margins, border and alignment. Inside the Iframe, the initial page of the guestbook is called up, throught the src attribute.
This way, the guestbook is operating within the Iframe. You might say the guestbook is completely unaware of the fact it's running in an Iframe, it might as well be running on it's own in a separate browser window. Whatever happens inside the Iframe, the rest of the site around the Iframe is untouched.
Hope this helps...