The JavaScript

Searching the user agent string for "Android" is the quickest method:

var ua = navigator.userAgent.toLowerCase();
var isAndroid = ua.indexOf("android") > -1; //&& ua.indexOf("mobile");
if(isAndroid) {
	// Do something!
	// Redirect to Android-site?
	window.location = 'http://android.davidwalsh.name';
}

The PHP

Again, we'll use PHP's strstr function to search for Android in the user agent:

$ua = strtolower($_SERVER['HTTP_USER_AGENT']);
if(stripos($ua,'android') !== false) { // && stripos($ua,'mobile') !== false) {
	header('Location: http://android.davidwalsh.name');
	exit();
}

Bonus!  .htaccess Detection

We can even use .htaccess directives to detect and react to Android devices!

RewriteCond %{HTTP_USER_AGENT} ^.*Android.*$
RewriteRule ^(.*)$ http://android.davidwalsh.name [R=301]

And there you have it:  three different Android device detection!  Have fun with your mobile development!