为什么这样还能提交过去呢？

<code>if(isset($_POST['yzm'])&&!empty($_POST['title'])&&!empty($_POST['content'])){
 echo "提交成功"
}</code>

<code><form method="post">
    <span name="yzm"></span>
    <input type="text" name="title">
    <textarea name="content"></textarea>
    <button type="submit"></button>
</form></code>

<code>if(isset($_POST['yzm'])&&!empty($_POST['title'])&&!empty($_POST['content'])){
 echo "提交成功"
}</code>

<code><form method="post">
    <span name="yzm"></span>
    <input type="text" name="title">
    <textarea name="content"></textarea>
    <button type="submit"></button>
</form></code>