PHP防止重复提交表单（helloweba网站经典实例）

<?php
  session_start();
  header("Content-Type:text/html;charset:utf8");
  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 "请不要重复提交";
           }else{
               echo "提交成功,value:".$_POST['web'];
           }

  }else{
?>
 <center>
 <form method="post" action="">
   <input type="hidden" name="token" value="<?php echo $_SESSION['token'] ?>" />
   <input type="text"  class="input" name="web" value="www.baidu.com" />
   <input type="submit"   class="btn"  value="提交" />
 </form>
</center>
<?php 
     } 
?>