<?php
if(isset($_GET['url'])){
$site = htmlentities(trim(stripslashes($_GET['url'])));}
else
{
$site="http://www.google.com";

}

?>
<iframe style="z-index:1" src="<?php echo "$site"?>" id="showframe" name="showframe" width = "1024px" height = "768px" align="center" scrolling="auto" frameborder="no" class="IFrames"></iframe>
şeklinde denermisin...