PHP 防止表单重复提交

<?php
/*
* PHP简单利用token防止表单重复提交
* 此处理方法纯粹是为了给初学者参考
*/
session_start();

var_dump( $_POST );
function set_token() {
     $_SESSION [ 'token' ] = md5(microtime(true));
     var_dump( $_SESSION [ 'token' ]);
}

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 [ 'test' ])){
    if (!valid_token()){
        echo "token error" ;
    } else {
        echo '成功提交，Value:' . $_POST [ 'test' ];
    }
}
?>
<form method= "post" action= "" >
    <input type= "hidden" name= "token" value= "<?php echo $_SESSION['token']?>" >
    <input type= "text" name= "test" value= "Default" >
    <input type= "submit" value= "提交" />
</form>