PHP HTML XMLHttpRequest.js

// JS
function request() {
    const xhr = new XMLHttpRequest();

    xhr.onreadystatechange = function() {
        if (xhr.readyState == XMLHttpRequest.DONE ) {
          console.log("xhr response", xhr.response)

           if (xhr.status == 200) { // success
           }
           else if (xhr.status == 400) { // error 400 (bad request)

           }
           else { // error

           }
        }
    };

    const data = {
      name: "Tristan",
      value: "dev",
    }

    xhr.open("POST", "./search.php");
    xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    xhr.send(JSON.stringify(data));
}

// PHP
<?php
$data = json_decode(file_get_contents('php://input'), true);
?>