Below is a useful script I used for detecting mobile browsers.

// Detects if browser is mobile or not
$mobile_browser = '0';
if(preg_match('/(up.browser|up.link|windows   ce|iemobile|mmp|symbian|smartphone|midp|wap|phone|vodafone|o2|pocket|mobile|pda|psp)/i',strtolower($_SERVER['HTTP_USER_AGENT']))){
 $mobile_browser++;
}
if(((strpos(strtolower($_SERVER['HTTP_ACCEPT']),'text/vnd.wap.wml')>0) or (strpos(strtolower($_SERVER['HTTP_ACCEPT']),'application/vnd.wap.xhtml+xml')>0)) or ((((isset($_SERVER['HTTP_X_WAP_PROFILE']) or isset($_SERVER['HTTP_PROFILE']) or isset($_SERVER['X-OperaMini-Features']) or isset($_SERVER['UA-pixels'])))))){
 $mobile_browser++;
}
$mobile_ua = strtolower(substr($_SERVER['HTTP_USER_AGENT'],0,4));
$mobile_agents = array('acs-','alav','alca','amoi','audi','aste','avan','benq','bird','blac','blaz','brew','cell','cldc','cmd-','dang','doco','eric','hipt','inno','ipaq','java','jigs','kddi','keji','leno','lg-c','lg-d','lg-g','lge-','maui','maxo','midp','mits','mmef','mobi','mot-','moto','mwbp','nec-','newt','noki','opwv','palm','pana','pant','pdxg','phil','play','pluc','port','prox','qtek','qwap','sage','sams','sany','sch-','sec-','send','seri','sgh-','shar','sie-','siem','smal','smar','sony','sph-','symb','t-mo','teli','tim-','tosh','tsm-','upg1','upsi','vk-v','voda','wap-','wapa','wapi','wapp','wapr','webc','winw','winw','xda','xda-');
if(in_array($mobile_ua,$mobile_agents)){
 $mobile_browser++;
}
if($mobile_browser>0){
 //mobile site
}else{
 //not mobile site
 header( 'Location: http://www.web_address.com/' ) ;
}