PHP submits a form rand without overwriting the previous one?

<code>$s1 = rand(1,9);
$s2 = rand(1,9);

if($_POST['daan']){
    if($_POST['daan'] == $s1+$s2){
        $txt = '正确';
    }else{
        $s = $s1 + $s2;
        $txt = '错,正确答案是' .$s;
    }
}
</code>

?>

<code><?=$s1?> + <?=$s2?> = <input type="input" name="daan" />
<?=isset($txt) ? $txt : '请回答'?>
<input type="submit" name="button" id="button" value="提交" /></code>

The code is as above. The current situation is that after submitting the form, the rand value is the value after submission, causing the answer to be wrong. What can I do to avoid overwriting the last rand value?

Reply content:

<code>$s1 = rand(1,9);
$s2 = rand(1,9);

if($_POST['daan']){
    if($_POST['daan'] == $s1+$s2){
        $txt = '正确';
    }else{
        $s = $s1 + $s2;
        $txt = '错,正确答案是' .$s;
    }
}
</code>

?>

<code><?=$s1?> + <?=$s2?> = <input type="input" name="daan" />
<?=isset($txt) ? $txt : '请回答'?>
<input type="submit" name="button" id="button" value="提交" /></code>

The code is as above. The current situation is that after submitting the form, the rand value is the value after submission, causing the answer to be wrong. What can I do to avoid overwriting the last rand value?

<code>session_start();
if($_POST['daan']){
    if($_POST['daan'] == $_SESSION['result']){
        $txt = '正确';
    }else{
        $txt = '错,正确答案是' .$_SESSION['result'];
    }
}else{
    $s1 = rand(1,9);
    $s2 = rand(1,9);
    $_SESSION['result'] = $s1 + $s2;
}</code>

After you submit the form, the page will refresh again, and $s1 and $s2 will re-obtain a random value.
The way to solve the problem is to not change the value when it is refreshed. You put it in session Just stay inside.

<code class="php">session_start();
$s1 = $_SESSION['s1'] = rand(1, 9);
$s2 = $_SESSION['s2'] = rand(1, 9);

//然后下面的判断用$_SESSION的值就行了</code>

When refreshing, $_SESSION[], like a variable, will get a new value.