<button type="button" class="btn btn-dark" onclick="countdown()">Start! <i class="fas fa-stopwatch"></i></button>
            <script>
               function countdown() {
                  $.ajax({
                    type: "POST",
                    url: 'countdown.php'
                    }
                  });
                }
            </script>
<iframe class="mt-1 ml-1" src="countdown.php" frameBorder="0" style="width: 13rem;"></iframe>

<?php
echo '
      <p>Time left: </p>
      <h2 id="countdown"><span id="time">/</span></h2>
      <script>
      var time = 20;
      var tick = new Audio("../sounds/tick.wav");
      var timer = setInterval(function(){
        if (time > 0) {
          time = time - 1;
          tick.play();
          document.getElementById("countdown").innerHTML = time;
        }
        if (time == 0) {
          var audio = new Audio("../sounds/zig.wav");
          audio.play();
        }
      },1000);
    }
    </script>
    ';
}
?>