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
      }
 ?>