Although the iframe tag makes itself out to be part of the regular DOM (document object model), it actually is more like an entirely new web-browser within your page. Because of this it is restricted by the cross-domain policy issue that causes many people headaches when trying to use the tag. This may not seem an issue at first, but if you try to use javascript to access the 2nd DOM within the iframe, you'll find a security exception is thrown.
Also PHP code executes on the server only, by the time the web-page is displayed, the PHP script has finished running, and the result has been downloaded to the web-browser. POST variables are not sent to the web-browser, so no you can't pass them to an iframe via javascript.
An answer to your situation is that you can actually achieve what you want by passing a posted variable as a GET style variable into the src of the iframe tag like so:
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<?php
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$url = 'sub_page.php?a=' . $_POST['a'];
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// Do some validation here.
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?>
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<iframe src="<?php echo $url; ?>"></iframe>
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It should be noted though, if you use this work-around, users to your site will be able to see the variable in the source code, also I've left a comment in there to remind you that you should always validate incoming variables to make sure they are not malicious.
Hope this helps!
Aimee.
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Edit: just saw that you had already posted another question about this a few weeks ago but got no replies, I know GET variables may not be the perfect answer, but when using the iframe tag, and all the quirks that come with it, the most consistently reliable method is to pass the variable to the src tag attribute, it should also be noted however that POST vars within the iframe itself stay in scope (if that helps).
Lastly have you considered ditching the iframe tag all together? if it's just scrolling and dynamic content loading you want, you can achieve that with a little bit of ajax and a div with scroll bars, which would remove this problem all together.
Aimee.
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