if(window.XMLHttpRequest) // Firefox
    http = new XMLHttpRequest();
  else if(window.ActiveXObject) // Internet Explorer
    http = new ActiveXObject("Microsoft.XMLHTTP");
  else { // XMLHttpRequest non supporté par le navigateur
    alert("Votre navigateur ne supporte pas les objets XMLHTTPRequest...");
    return;
  }

  http.open("POST", "phpToJs/findRdv.inc.php", true);

  http.onreadystatechange = function() {
  if(http.readyState == 4 && http.status == 200)
    var response = http.responseText;
    D.innerHTML = response;
  }

// PHP findRdv.inc.php
echo 'hello world';