php如何实现跨域请求

XMLHttpRequest cannot load http://server.runoob.com/server.php. No 'Access-Control-Allow-Origin' header is present on the requested resource.Origin 'http://client.runoob.com' is therefore not allowed access.

header('Access-Control-Allow-Origin:http://client.runoob.com');

$origin = isset($_SERVER['HTTP_ORIGIN'])? $_SERVER['HTTP_ORIGIN'] : '';  
  $allow_origin = array(  
    'http://client1.runoob.com',  
    'http://client2.runoob.com'  );  
  if(in_array($origin, $allow_origin)){  
    header('Access-Control-Allow-Origin:'.$origin);       }

header('Access-Control-Allow-Origin:*');