php通过http post发送json数据

function http_post_data($url, $data_string) {
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array(
        'Content-Type: application/json; charset=utf-8',
        'Content-Length: ' . strlen($data_string))
    );
    ob_start();
    curl_exec($ch);
    $return_content = ob_get_contents();
    ob_end_clean();

    $return_code = curl_getinfo($ch, CURLINFO_HTTP_CODE);
    return array($return_code, $return_content);
}

$url  = "http://xx.xx.cn";
$data = json_encode(array('a'=>1, 'b'=>2)); 

$res = http_post_data($url, $data);