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