这是我的代码，用于显示 otp 的倒计时器，但它仅显示静态值，并且不会像预期那样每秒更新。

<?php 
// Set the target end time (in this example, 10 minutes from now) 
$endTime = time() + (10 * 60);  
 
// Calculate the remaining time 
$remainingTime = $endTime - time(); 
 
// Display the remaining time 
echo "<span id='countdown'></span>"; 
 
?> 
 
<script> 
// Display the remaining time in minutes and seconds 
function displayCountdown() { 
  var remainingTime = <?php echo $remainingTime; ?>; 
  var minutes = Math.floor(remainingTime / 60); 
  var seconds = remainingTime - (minutes * 60); 
  document.getElementById("countdown").innerHTML = minutes + "m " + seconds + "s"; 
  remainingTime--; 
  if (remainingTime < 0) { 
    clearInterval(interval); 
    document.getElementById("countdown").innerHTML = "Time's up!"; 
  } 
} 
var interval = setInterval(displayCountdown, 1000); 
</script>

请指出我缺少什么。