Any disadvantages to using iframes

Question

I've been advised to use an iframe to share a form between 2 websites, but I always got the impression that iframes are bad or that they shouldn't be used. Not sure I even remember what gave me this impression. So can someone clarify if using an iframe is bad practice or not, and if it is bad practice, why?

Answer 1

If you are using PHP, it's better to put the form in it's own file, and include it.

http://php.net/manual/en/function.include.php

Answer 2

iFrames allow you to share a limited amount of information (client-side) across domains. Many web ads are loaded within iFrames.

iFrames are heavy elements (the iFrame is the most expensive element to add to an HTML page) so adding many will be slow but one is usually well within acceptable.