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http://stackoverflow.com/questions/4117555/simplest-way-to-detect-a-mobile-device

<?php

function isMobile() {
    return preg_match("/(android|avantgo|blackberry|bolt|boost|cricket|docomo|fone|hiptop|mini|mobi|palm|phone|pie|tablet|up\.browser|up\.link|webos|wos)/i", $_SERVER["HTTP_USER_AGENT"]);
}


UPDATED VERSION:
return preg_match("/(android|webos|avantgo|iphone|ipad|ipod|blackberry|iemobile|bolt|bo‌​ost|cricket|docomo|fone|hiptop|mini|opera mini|kitkat|mobi|palm|phone|pie|tablet|up\.browser|up\.link|webos|wos)/i", $_SERVER["HTTP_USER_AGENT"]);




// Use the function
if(isMobile()){
    // Do something for only mobile users
}
else {
    // Do something for only desktop users
}





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To redirect a user to your mobile site, I would do this
// Create the function, so you can use it
function isMobile() {
    return preg_match("/(android|avantgo|blackberry|bolt|boost|cricket|docomo|fone|hiptop|mini|mobi|palm|phone|pie|tablet|up\.browser|up\.link|webos|wos)/i", $_SERVER["HTTP_USER_AGENT"]);
}
// If the user is on a mobile device, redirect them
if(isMobile()){
    header("Location: http://m.yoursite.com/");
}