Access to XMLHttpRequest at 'http://localhost/MySQL_pracs/InsertUser.php' from origin 'http://localhost:4200' has been blocked by CORS policy: Request header field content-type is not allowed by Access-Control-Allow-Headers in preflight response.
//Access to XMLHttpRequest at 'http://localhost/[api path].php' from origin 'http://localhost:4200' has been blocked by CORS policy: Request header field content-type is not allowed by Access-Control-Allow-Headers in preflight response.
//The error is simply saying that "Content-Type" is missing from "Access-Control-Allow-Headers".
//Therefore we need to add "Content-Type" to "Access-Control-Allow-Headers".
<?php
header('Access-Control-Allow-Headers: Content-Type');
-----
?>