浏览器在发起请求是，在http请求头里面有一个它名为 User Agent，简称 UA。里面包含了客户端基础信息的字符串。通过它可以方便的获取客户端的操作系统，语言，浏览器和版本信息。

php常规判断设备类型，可以读取这里的信息，作为判断依据:

        if (!function_exists("isMobile")) {
            function isMobile()
            {
                    $user_agent = $_SERVER['HTTP_USER_AGENT'];
                    $mobile_agents = array ('iphone','android','phone','mobile','wap','netfront','java','opera mobi',
                            'opera mini','ucweb','windows ce','symbian','series','webos','sony','blackberry','dopod',
                            'nokia','samsung','palmsource','xda','pieplus','meizu','midp','cldc','motorola','foma',
                            'docomo','up.browser','up.link','blazer','helio','hosin','huawei','novarra','coolpad',
                            'techfaith','alcatel','amoi','ktouch','nexian','ericsson','philips','sagem','wellcom',
                            'bunjalloo','maui','smartphone','iemobile','spice','bird','zte-','longcos','pantech',
                            'gionee','portalmmm','jig browser','hiptop','benq','haier','^lct','320x320','240x320',
                            '176x220','windows phone','cect','compal','ctl','lg','nec','tcl','daxian','dbtel','eastcom',
                            'konka','kejian','lenovo','mot','soutec','sgh','sed','capitel','panasonic','sonyericsson',
                            'sharp','panda','zte','acer','acoon','acs-','abacho','ahong','airness','anywhereyougo.com',
                            'applewebkit/525','applewebkit/532','asus','audio','au-mic','avantogo','becker','bilbo',
                            'bleu','cdm-','danger','elaine','eric','etouch','fly ','fly_','fly-','go.web','goodaccess',
                            'gradiente','grundig','hedy','hitachi','htc','hutchison','inno','ipad','ipaq','ipod',
                            'jbrowser','kddi','kgt','kwc','lg ','lg2','lg3','lg4','lg5','lg7','lg8','lg9','lg-','lge-',
                            'lge9','maemo','mercator','meridian','micromax','mini','mitsu','mmm','mmp','mobi','mot-',
                            'moto','nec-','newgen','nf-browser','nintendo','nitro','nook','obigo','palm','pg-',
                            'playstation','pocket','pt-','qc-','qtek','rover','sama','samu','sanyo','sch-','scooter',
                            'sec-','sendo','sgh-','siemens','sie-','softbank','sprint','spv','tablet','talkabout',
                            'tcl-','teleca','telit','tianyu','tim-','toshiba','tsm','utec','utstar','verykool','virgin',
                            'vk-','voda','voxtel','vx','wellco','wig browser','wii','wireless','xde','pad','gt-p1000');
                    $ismobile = false;
                    foreach ($mobile_agents as $device) {
                            if (stristr($user_agent, $device)) {
                                    $ismobile = true;
                                    break;
                            }
                    }
                    return $ismobile;
            }
    }