iframe with src=xyz.php fails accessibility check

[Patrick666]

I have an iframe in my home page (index.php) .  The iframe src is a PHP page(start.php).  The user may link to another topic, this will load in page2.php to the iframe (or page3,4 etc.).
All this works fine and passes HTML validation checks.
When I try to get the home page through accessibility checks it fails.
The complaint (6.2) is that the iframe src must be an HTML page.
If I set the src to an HTML page it passes.
But I am doing PHP stuff in start.php and an HTML page will not do.
The only thing I can think of is to set the iframe src to an HTML page which has no content but has a refresh with zero timeout to my PHP page.
Any better suggestions or comments.
I just read thread where hanlonj suggests not using iframes.
hanlonj, if your there, your comments would be welcome.
If I scrap the iframe it will mean a complete redesign, and I have got a terrible feeling that is exactly what you are going to say.
Although all my code works, I would like it to pass accessibility.
Patrick666

[mainewoods]

does the start.php  page return a complete html page with the HTML tag and the body tag and stuff?  That's probably what the validator is complaining about.  It should also have a doctype tag as well to validate.