php获取当前http或者https协议

if(isset($_SERVER['HTTP_X_CLIENT_SCHEME'])){
  $scheme = $_SERVER['HTTP_X_CLIENT_SCHEME'] . '://';
}elseif(isset($_SERVER['REQUEST_SCHEME'])){
  $scheme = $_SERVER['REQUEST_SCHEME'] . '://';
}else{
  $scheme = 'http://';
}
echo $scheme;