You need three total pages -- linkpage.php, iframe.php, and samplepage.html.
In linkpage.html, you will include the following link somewhere on the page:
HTML Code:
<a href='iframe.php?url=samplepage.html'>Click Here</a>
When clicked on, the link will open iframe.php. Where you want the iframe to display, you will place the following code:
PHP Code:
<?php
echo "<iframe src='http://".$_GET['url']."' />";
?>
This will call the url referenced in the address.