I couldn’t believe that there wasn’t already a solution for this, so I made my own. This solution uses a bit of php, javascript, and of course, flash. Please keep in mind that I cannot guarantee the reliability and/or quality of this solution, all I know is that it works for me, with IIS and PHP 5 (also works with PHP 4).

I tried to make this flexible, so the code that I inserted in the page that contains the page is:
<div id="container"><?php $flash_file = "flash_file.swf"; include("insert_flash.php");?></div>
And then I made an file called ‘insert_flash.php’. The code I put in was:
<div id="flashcontent">
<p>My alternate text in case the reader doesn't have flash.</p>
</div>
<script type="text/javascript">
if ("<?php echo $flash_file;?>" != ""){
dwidth = document.getElementById('container').offsetWidth;
var width = <?php $headerDims = getimagesize($flash_file);$headerWidth = $headerDims[0];echo $headerWidth;?>;
var height = <?php echo $headerHeight = $headerDims[1];$headerHeight;?>;
if (width > height) {
var percentage = (dwidth / width);
}
else {
var percentage = (dwidth / height);
}
}
var width = Math.floor(width * percentage)-2;
var height = Math.floor(height * percentage)-2;
var so = new SWFObject("<?php echo $flash_file;?>", "mymovie",width,height, "6", "#ffffff");
so.addParam("wmode", "opaque");
so.write("flashcontent");
window.onresize = function(){ //this is if the window gets resized by the user, so you don't have to reload the page on every reload
dwidth = document.getElementById('container').offsetWidth;
var width = <?php $headerDims = getimagesize($flash_file);$headerWidth = $headerDims[0];echo $headerWidth;?>;
var height = <?php echo $headerHeight = $headerDims[1];$headerHeight;?>;
if (width > height) {
var percentage = (dwidth / width);
}
else {
var percentage = (dwidth / height);
}
var width = Math.floor(width * percentage)-2;
var height = Math.floor(height * percentage)-2;
var so = new SWFObject("<?php echo $flash_file;?>", "mymovie",width,height, "6", "#ffffff");
so.addParam("wmode", "opaque");
so.write("flashcontent");
}
</script>
Try it out and play with it, the only thing I really experimented with was the ‘-2′ when setting the width and height variables. I’m sure this could be adjusted by setting the properties of the container and flashcontent divs.

Any improvements to suggest? I did try to put the ‘window.onresize’ function into a separate function, so I wouldn’t have to reuse code, but it didn’t work, for some reason.