php - Print form every 30 minutes

I want to echo a form every 30 minutes each day for 18 repetition each day and this repetition will appear each 30 minutes and when the form appear the user will check the checkbox and submit the form and when he submits the form the form disappears and this will be do it 18 repetition per day.

I tried this code but it not show me any thing it still running without any output:

$dailydatetime=date('Y-m-d H:i:s');
$echo_time = time();
$interval = 1*60;
  while(true){
    if ($echo_time + $interval >= time()){
      echo "<form method='Post' action=''>";
      echo"<input type='checkbox' name='check' value=''>"  ;
      echo"<input type='submit' name='add' value='Check'>";  
      echo"</form>";  
      $echo_time = time(); 
    }

  }
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