We can direct people to our mobile site using JavaScript. What this following code does is look for a screen size and then redirects the user.

<script type="text/javascript">
if (screen.width <= 699) {
document.location = "mymobilesite.html";
}

The following code will target iPhone and iPods to your mobile website.

if ((navigator.userAgent.match(/iPhone/i)) || (navigator.userAgent.match(/iPod/i))) {
   location.replace("http://m/mywebsite.com");
}
</script>

Put this link code in your header so that Google knows where your mobile site is located.

<link rel="alternate" href="http://mywebsite/mobile/" media="only screen and (max-width: 640px)">

More detailed redirects in JavaScript

<script type="text/javascript">
        var getStr = window.location.search.substr(1);
        var getArray = getStr.split ("&");
        var get = {};

        for ( var i = 0; i < getArray.length; i++) {
        var tmpArray = getArray[i].split("=");
        get[tmpArray[0]] = tmpArray[1];
        }
       if (get.m == "yes") {
                window.location = "http://m.example.com"
        }
        else if (get.m == "no") {
                window.location = "http://www.example.com";
        }

Redirect to Mobile site using PHP

<?php if(strstr($_SERVER['HTTP_USER_AGENT'],'iPhone') || strstr($_SERVER['HTTP_USER_AGENT'],'iPod'))
{
  header('Location: http://yoursite.com/iphone');
  exit();
}?>