php接口怎么判断重复提交

SESSION_START(); 
$_SESSION['is_submit'] = 0;
<form id="reg" action="post.php" method="post"> 
    <p>用户名：<input type="text" class="input" name="username" id="user"></p> 
    <p>密   码：<input type="password" class="input" name="password" id="pass"></p><p><span>立即学习</span>“<a href="https://pan.quark.cn/s/7fc7563c4182" style="text-decoration: underline !important; color: blue; font-weight: bolder;" rel="nofollow" target="_blank">PHP免费学习笔记（深入）</a>”；</p> 
    <p>E-mail：<input type="text" class="input" name="email" id="email"></p> 
    <p><input type="submit" name="submit" class="btn" value="提交注册"/></p> 
</form>

SESSION_START(); 
if (isset($_POST['submit'])) { 
    if ($_SESSION['is_submit'] == '0') { 
        $_SESSION['is_submit'] = '1'; 
        echo "代码块，要做的事，代码...<a onclick='history.go(-1);' href='javascript:void(0)'>返回</a>"; 
    } else { 
        echo "请不用重复提交<a href='index.php'>PHP+SESSION防止表单重复提交</a>"; 
    } 
}