detect mobile device and redirect according to screen height using only one index.php file
===

After detecting if my web site is used on a mobile device of not by parsing the USER_AGENT string, 
I would like to redirect the user to the pages optimized for resolutions with height >= 1024 (for example 9.7", 10.1" tablets)
or redirect the user to pages optimized for resolutions < 1024 (for example smarphones and 5", 7", 8" tablets)

To acheive this I need to get the screen resolution with javascript.

We know that PHP (the USER_AGENT parser script) is executed before javascript but I need to get the screen resolution before the parser. 
As I know that a web server executes firstly INDEX.HTML and then INDEX.PHP I wrote a Index.html file which gets the screen width and pass it via a form with POST (to keep the address bar clean rather than $_GET) to Index.php which then loads (includes) the corrects web page.
	
	INDEX.HTML
	<body onload="document.form.submit();">
		<form name="form" action="index.php" method="POST">
			<input type="hidden" id="height" name="height" value="" />
		</form>
		<script type="text/javascript">
			document.getElementById('height').value = screen.height;
		</script>
	</body>
	
	INDEX.PHP
	<?php
		$useragent=$_SERVER['HTTP_USER_AGENT'];
		
		# If it's a mobile device...
		if(preg_match('/android.+mobile|avantgo|bada\/|blackberry|blazer|compal|elaine|fennec|hiptop|iemobile|ip(hone|od)|iris|kindle|lge |maemo|meego.+mobile|midp|mmp|netfront|opera m(ob|in)i|palm( os)?|phone|p(ixi|re)\/|plucker|pocket|psp|series(4|6)0|symbian|treo|up\.(browser|link)|vodafone|wap|windows (ce|phone)|xda|xiino/i',$useragent)||preg_match('/1207|6310|6590|3gso|4thp|50[1-6]i|770s|802s|a wa|abac|ac(er|oo|s\-)|ai(ko|rn)|al(av|ca|co)|amoi|an(ex|ny|yw)|aptu|ar(ch|go)|as(te|us)|attw|au(di|\-m|r |s )|avan|be(ck|ll|nq)|bi(lb|rd)|bl(ac|az)|br(e|v)w|bumb|bw\-(n|u)|c55\/|capi|ccwa|cdm\-|cell|chtm|cldc|cmd\-|co(mp|nd)|craw|da(it|ll|ng)|dbte|dc\-s|devi|dica|dmob|do(c|p)o|ds(12|\-d)|el(49|ai)|em(l2|ul)|er(ic|k0)|esl8|ez([4-7]0|os|wa|ze)|fetc|fly(\-|_)|g1 u|g560|gene|gf\-5|g\-mo|go(\.w|od)|gr(ad|un)|haie|hcit|hd\-(m|p|t)|hei\-|hi(pt|ta)|hp( i|ip)|hs\-c|ht(c(\-| |_|a|g|p|s|t)|tp)|hu(aw|tc)|i\-(20|go|ma)|i230|iac( |\-|\/)|ibro|idea|ig01|ikom|im1k|inno|ipaq|iris|ja(t|v)a|jbro|jemu|jigs|kddi|keji|kgt( |\/)|klon|kpt |kwc\-|kyo(c|k)|le(no|xi)|lg( g|\/(k|l|u)|50|54|\-[a-w])|libw|lynx|m1\-w|m3ga|m50\/|ma(te|ui|xo)|mc(01|21|ca)|m\-cr|me(di|rc|ri)|mi(o8|oa|ts)|mmef|mo(01|02|bi|de|do|t(\-| |o|v)|zz)|mt(50|p1|v )|mwbp|mywa|n10[0-2]|n20[2-3]|n30(0|2)|n50(0|2|5)|n7(0(0|1)|10)|ne((c|m)\-|on|tf|wf|wg|wt)|nok(6|i)|nzph|o2im|op(ti|wv)|oran|owg1|p800|pan(a|d|t)|pdxg|pg(13|\-([1-8]|c))|phil|pire|pl(ay|uc)|pn\-2|po(ck|rt|se)|prox|psio|pt\-g|qa\-a|qc(07|12|21|32|60|\-[2-7]|i\-)|qtek|r380|r600|raks|rim9|ro(ve|zo)|s55\/|sa(ge|ma|mm|ms|ny|va)|sc(01|h\-|oo|p\-)|sdk\/|se(c(\-|0|1)|47|mc|nd|ri)|sgh\-|shar|sie(\-|m)|sk\-0|sl(45|id)|sm(al|ar|b3|it|t5)|so(ft|ny)|sp(01|h\-|v\-|v )|sy(01|mb)|t2(18|50)|t6(00|10|18)|ta(gt|lk)|tcl\-|tdg\-|tel(i|m)|tim\-|t\-mo|to(pl|sh)|ts(70|m\-|m3|m5)|tx\-9|up(\.b|g1|si)|utst|v400|v750|veri|vi(rg|te)|vk(40|5[0-3]|\-v)|vm40|voda|vulc|vx(52|53|60|61|70|80|81|83|85|98)|w3c(\-| )|webc|whit|wi(g |nc|nw)|wmlb|wonu|x700|yas\-|your|zeto|zte\-/i',substr($useragent,0,4))) {
			# If the device screen height is => 1024 it's a tablet 8", 9.7" or 10.1"
			if ($_POST['height'] >= 1024) {
				include ('include.tablet.php');
			}
			# If the device screen height is < 1024 it's a smartphone or a tablet 5" o 7")...
			else {
				include ('include.smartphone.php');
			}
		}
		# If it's a desktop PC...
		else {
			  	include ('include.index.php');
			}
		}
	?>	

Now I'm trying to do the same thing without the HTML file. I wrote a index.php file which gets screen.width and send it to itself with $_SERVER['PHP_SELF'] 
so that the PHP code may redirect the user to the correct page.
	
	<body onload="document.form.submit();">
		<form name="form" action="<?php echo $_SERVER['PHP_SELF']; ?>" method="POST">
			<input type="hidden" id="height" name="height" value="" />
		</form>
		<script type="text/javascript">
			document.getElementById('height').value = screen.height;
		</script>
		<?php if(isset($_POST['height'])) {	echo $_POST['height']; } ?>
	</body>
	
The issue is that the method works, but the browser (Firefox) take 1 or 2 seconds to run the page and return the result. Why?

I know that I could use a javascript parser and then redirect to the correct page with: 'window.location.href' but **I want my homepage to be always index.php** and not start with index.php which redirects to tablet.php or smartphone.php. I'm trying to keep the code elegant.

Any suggestion?