javascript - No 'Access-Control-Allow-Origin' header is present error?

one text

I have next ajax request usnig jquery in my react app, here is this request:

index.js

$.ajax({
  type: "POST",
  url : 'http://localhost/my-app/server/send.php',
  "async": true,
  "crossDomain": true,
  "headers": {
    "accept": "application/json",
    "Access-Control-Allow-Origin":"*"
  }
}, function(data) {
  alert(data, "success!")
})

send.php

<?
header("Access-Control-Allow-Origin: *");
echo "...response...";

But my ajax call raises an error in console:

Access to XMLHttpRequest at 'http://localhost/my-app/server/send.php' from origin 
'http://localhost:3000' has been blocked by CORS policy: Response to preflight request doesn't pass 
 access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource

But I send headers, what is wrong?

Source