In PHP HTTP_USER_AGENT is used to judge whether it is the function code of mobile terminal

Sometimes it's useful in 1 or 2 situations

function is_mobile_request()   
    {   
      $_SERVER['ALL_HTTP'] = isset($_SERVER['ALL_HTTP']) ? $_SERVER['ALL_HTTP'] : '';   
      $mobile_browser = '0';   
      if(preg_match('/(up.browser|up.link|mmp|symbian|smartphone|midp|wap|phone|iphone|ipad|ipod|android|xoom)/i', strtolower($_SERVER['HTTP_USER_AGENT'])))   
        $mobile_browser++;   
      if((isset($_SERVER['HTTP_ACCEPT'])) and (strpos(strtolower($_SERVER['HTTP_ACCEPT']),'application/vnd.wap.xhtml+xml') !== false))   
        $mobile_browser++;   
      if(isset($_SERVER['HTTP_X_WAP_PROFILE']))   
        $mobile_browser++;   
      if(isset($_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') !== false)   
        $mobile_browser++;   
      // Pre-final check to reset everything if the user is on Windows   
      if(strpos(strtolower($_SERVER['HTTP_USER_AGENT']), 'windows') !== false)   
        $mobile_browser=0;   
      // But WP7 is also Windows, with a slightly different characteristic   
      if(strpos(strtolower($_SERVER['HTTP_USER_AGENT']), 'windows phone') !== false)   
        $mobile_browser++;   
      if($mobile_browser>0)   
        return true;   
      else  
        return false;   
    }

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php determines the user's browser type based on HTTP_USER_AGENT

function browsers(){
   global $HTTP_USER_AGENT ;
   if (isset($HTTP_USER_AGENT)){
    $sAgent = $HTTP_USER_AGENT;
   }else{
    $sAgent = $_SERVER['HTTP_USER_AGENT'];
   }
   if (strpos($sAgent,'MSIE') !== false && strpos($sAgent,'mac') === false && strpos($sAgent,'Opera') === false){
    $iVersion = (float)substr($sAgent,strpos($sAgent,'MSIE') + 5,3);
    return ($iVersion >= 5.5) ;
   }else if (strpos($sAgent,'Gecko/') !== false){
    $iVersion = (int)substr($sAgent,strpos($sAgent,'Gecko/') + 6,8);
    return ($iVersion >= 20030210) ;
   }else{
    return false;
   }
  }