PHP如何防止用户重复提交表单

< ?php
/  PHP简单利用token防止表单重复提交 /
session_start();
header("Content-Type: text/html;charset=utf-8");
function set_token() {
  $_SESSION['token'] = md5(microtime(true));
}
function valid_token() {
  $return = $_REQUEST['token'] === $_SESSION['token'] ? true: false;
  set_token();
  return $return;
}
//如果token为空则生成一个token 
if(!isset($_SESSION['token']) || $_SESSION['token']=='') { 
  set_token(); 
}  
if(isset($_POST['web'])){ 
  if(!valid_token()){ 
    echo "token error，请不要重复提交！"; 
  }else{ 
  echo '成功提交，Value:'.$_POST['web']; 
  } 
}else{ 
?> 
  <form method="post" action=""> 
  <input type="hidden" name="token" value="<?php echo $_SESSION['token']?>"> 
  <input type="text" class="input" name="web" value=".jb51."> 
  <input type="submit" class="btn" value="提交" /> 
  </form> 
<?php 
} 
?>