//跨域访问的时候才会存在此字段

$origin = isset($_SERVER['HTTP_ORIGIN']) ? $_SERVER['HTTP_ORIGIN'] : ''; 

$allow_origin = array(

    'http://www.a.com',

    'http://www.b.com'

); 

if(in_array($origin, $allow_origin)){

    header('Access-Control-Allow-Origin:'.$origin);

    header('Access-Control-Allow-Methods:POST, GET');

    header('Access-Control-Allow-Headers:x-requested-with,content-type');

}