AMP Access Control Allow Source Origin header Issue

Please try with following code

if(!empty($_POST)){
        $domain_url = (isset($_SERVER['HTTPS']) ? "https" : "http") . "://$_SERVER[HTTP_HOST]";
        header("Content-type: application/json");
        header("Access-Control-Allow-Credentials: true");
        header("Access-Control-Allow-Origin: ". str_replace('.', '-','https://example.com') .".cdn.ampproject.org");
        header("AMP-Access-Control-Allow-Source-Origin: " . $domain_url);
        header("Access-Control-Expose-Headers: AMP-Access-Control-Allow-Source-Origin");
        header("AMP-Redirect-To: https://example.com/thankyou.amp.html");
        header("Access-Control-Expose-Headers: AMP-Redirect-To, AMP-Access-Control-Allow-Source-Origin"); 
        echo json_encode(array('successmsg'=>'data post'));
        exit;
}

Please make sure that domain url should be https

Replace https://example.com/ to your desired url