How do I implement basic "Long Polling"?

<?php

// Prevent PHP execution timeout during long waits
set_time_limit(0);

// Define a function to simulate getting new data
function getNewData()
{
  // Simulate data arriving after a few checks
  static $checkCount = 0;
  $checkCount++;
  if ($checkCount > 5) {
    return "New data is available!";
  }
  return null;
}

// Set maximum time the server should keep the connection open
$maxWaitTime = 30; // seconds
$startTime = time();

// Keep the connection open until new data is available or the maximum wait time is reached
while (true) {
  // Check for new data
  $newData = getNewData();
  if ($newData) {
    // Send new data to the client
    echo $newData;
    // Flush output buffers to ensure data is sent immediately
    ob_flush();
    flush();
    break;
  }

  // Check if the maximum wait time has been reached
  $waitTime = time() - $startTime;
  if ($waitTime >= $maxWaitTime) {
    break;
  }

  // Sleep for a short period of time before checking for new data again
  usleep(100000); // 100ms
}
?>

function longPoll() {
  fetch('long_polling.php')
    .then(response => response.text())
    .then(data => {
      console.log('Received:', data);
      // Handle the new data here
      // Immediately initiate the next request to maintain the cycle
      longPoll();
    })
    .catch(error => {
      console.error('Polling failed:', error);
      // Retry after a delay on error
      setTimeout(longPoll, 5000);
    });
}

// Start the long polling cycle
longPoll();