php mysql防止快速表单提交的重复插入

I'm testing with this code:

我正在测试这段代码:

<?php
$db = new PDO('mysql:host=localhost;dbname=test', 'root', 'root');

if(filter_has_var(INPUT_POST, 'submit')) {

  $r = $db->query('SELECT * FROM test WHERE value="'.$_POST['value'].'"');
  $n = $r->rowCount();

  for ($i=0; $i<=10000000; $i++) {
    // do nothing
  }

  if  (!$n) {
    $db->query('INSERT INTO test (id, value) VALUES (NULL, "'.$_POST['value'].'")');
  }

}
?>

<form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
  <input type="text" name="value" value="">
  <input type="submit" name="submit" value="Submit">
</form>
<?ph