uvod/bmi-rest.html

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Výpočet BMI</title>
<script>

window.onload = function() {
  var button = document.getElementById("btn");
  button.onclick = async function() {
    let response = await fetch("bmi-service.php", {
      method: "POST", 
      headers: {
        "Content-Type": "application/json",
      },
      body: JSON.stringify({
        vaha: document.getElementById("vaha").value,
        vyska: document.getElementById("vyska").value
      })
    });

    let data = await response.json();
    let result = document.getElementById("result");
    result.innerHTML = data.bmi;
  };
}

</script>
  </head>
  <body>
    <h1>Výpočet BMI</h1>

    Váha: <input id="vaha"><br>
    Výška: <input id="vyska"><br>

    <button id="btn">Spočítej BMI</button>  

    <h2>Vaše BMI je <span id="result"></span></h2>

  </body>
</html>