$mobile_browser = ’0′;

if(preg_match(‘/(up.browser|up.link|mmp|symbian|smartphone|midp|wap|phone)/i’,
strtolower($_SERVER['HTTP_USER_AGENT']))){
$mobile_browser++;
}

if((strpos(strtolower($_SERVER['HTTP_ACCEPT']),‘application/vnd.wap.xhtml+xml’)>0) or
((isset($_SERVER['HTTP_X_WAP_PROFILE']) or isset($_SERVER['HTTP_PROFILE'])))){
$mobile_browser++;
}

$mobile_ua = strtolower(substr($_SERVER['HTTP_USER_AGENT'],0,4));
$mobile_agents = array(
‘w3c ‘,‘acs-’,‘alav’,‘alca’,‘amoi’,‘audi’,‘avan’,‘benq’,‘bird’,‘blac’,
‘blaz’,‘brew’,‘cell’,‘cldc’,‘cmd-’,‘dang’,‘doco’,‘eric’,‘hipt’,‘inno’,
‘ipaq’,‘java’,‘jigs’,‘kddi’,‘keji’,‘leno’,‘lg-c’,‘lg-d’,‘lg-g’,‘lge-’,
‘maui’,‘maxo’,‘midp’,‘mits’,‘mmef’,‘mobi’,‘mot-’,‘moto’,‘mwbp’,‘nec-’,
‘newt’,‘noki’,‘oper’,‘palm’,‘pana’,‘pant’,‘phil’,‘play’,‘port’,‘prox’,
‘qwap’,‘sage’,‘sams’,‘sany’,‘sch-’,‘sec-’,‘send’,‘seri’,‘sgh-’,‘shar’,
‘sie-’,‘siem’,‘smal’,‘smar’,‘sony’,‘sph-’,‘symb’,‘t-mo’,‘teli’,‘tim-’,
‘tosh’,‘tsm-’,‘upg1′,‘upsi’,‘vk-v’,‘voda’,‘wap-’,‘wapa’,‘wapi’,‘wapp’,
‘wapr’,‘webc’,‘winw’,‘winw’,‘xda’,‘xda-’);

if(in_array($mobile_ua,$mobile_agents)){
$mobile_browser++;
}
if (strpos(strtolower($_SERVER['ALL_HTTP']),‘OperaMini’)>0) {
$mobile_browser++;
}
if (strpos(strtolower($_SERVER['HTTP_USER_AGENT']),‘windows’)>0) {
$mobile_browser=0;
}

if($mobile_browser>0){
// do something
} else {
// do something else
}
?>

Aprendi isto aqui: http://dev.mobi/article/lightweight-device-detection-php

Espero que seja muito útil! Um grande abraço e até a próxima!