думаю суть понятна

if(stristr($_SERVER[\'HTTP_USER_AGENT\'],\'windows\') || stristr($_SERVER[\'HTTP_USER_AGENT\'],\'linux\') ||
stristr($_SERVER[\'HTTP_USER_AGENT\'],\'macintosh\') || stristr($_SERVER[\'HTTP_USER_AGENT\'],\'unix\') ||
stristr($_SERVER[\'HTTP_USER_AGENT\'],\'macos\') || stristr($_SERVER[\'HTTP_USER_AGENT\'],\'bsd\'))
{
require_once($rootpath . \'web/head.php\');
}else{
require_once($rootpath . \'incfiles/t_head.php\');
}