本文通过设置Access-Control-Allow-Origin来实现跨域。
例如：客户端的域名是client.runoob.com，而请求的域名是server.runoob.com。
如果直接使用ajax访问，会有以下错误：
XMLHttpRequest cannot load

' target='_blank'>[/reply]' target='_blank'>' target='_blank'>[/reply][/reply]' target='_blank'>' target='_blank'>[/reply]' target='_blank'>' target='_blank'>[/reply][/reply][/reply]. No 'Access-Control-Allow-Origin' header is present on the requested resource.Origin '' target='_blank'>[/reply]' target='_blank'>' target='_blank'>[/reply][/reply]' is therefore not allowed access.
1、允许单个域名访问
指定某域名（' target='_blank'>[/reply]' target='_blank'>' target='_blank'>[/reply][/reply]）跨域访问，则只需在' target='_blank'>[/reply]' target='_blank'>' target='_blank'>[/reply][/reply]' target='_blank'>' target='_blank'>[/reply]' target='_blank'>' target='_blank'>[/reply][/reply][/reply]文件头部添加如下代码：
header('Access-Control-Allow-Origin:' target='_blank'>[/reply]' target='_blank'>' target='_blank'>[/reply][/reply]');
2、允许多个域名访问
指定多个域名（' target='_blank'>[/reply]、' target='_blank'>[/reply]等）跨域访问，则只需在' target='_blank'>[/reply]' target='_blank'>' target='_blank'>[/reply][/reply]' target='_blank'>' target='_blank'>[/reply]' target='_blank'>' target='_blank'>[/reply][/reply][/reply]文件头部添加如下代码：
$origin = isset($_SERVER['HTTP_ORIGIN'])? $_SERVER['HTTP_ORIGIN'] : '';

$allow_origin = array(
'' target='_blank'>[/reply]',
'' target='_blank'>[/reply]'
);

if(in_array($origin, $allow_origin)){
header('Access-Control-Allow-Origin:'.$origin);
}
3、允许所有域名访问
允许所有域名访问则只需在' target='_blank'>[/reply]' target='_blank'>' target='_blank'>[/reply][/reply]' target='_blank'>' target='_blank'>[/reply]' target='_blank'>' target='_blank'>[/reply][/reply][/reply]文件头部添加如下代码：
header('Access-Control-Allow-Origin:*');