我如何在过期时间后将字符串设置为零

我正在创建一个登录/注册脚本，用户必须使用令牌激活他的帐户。现在我有一个函数，它调用一个随机字符串。

class.functions.php

public static function generateRandomString($length = 200) {
        $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
        $charactersLength = strlen($characters);
        $randomString = '';
        for ($i = 0; $i < $length; $i++) {
            $randomString .= $characters[rand(0, $charactersLength - 1)];
        }
        return $randomString;
    }

这将输出如下内容：jTq7tH4dKy66dmnhRev5EjbZDZygN1...

现在我的问题是，如何在24小时后将此字符串设置为空，以便用户必须再次激活他的帐户

我的想法是关于我的数据库中的update query，如下所示：

$statusN = "N" and $statusY = "Y"

数据库中的表字段：

`user_status` ENUM('Y','N') NOT NULL DEFAULT 'N',

sql语句：

$sql = "UPDATE formular SET user_status='" . $statusY . "' WHERE user_status='" . $statusN . "'";

但我不知道如何随时间推移

这是用户数据和activation_token 的插入

register.php

$activationToken = functions::generateRandomString();
    $stmt = $conn->prepare("INSERT INTO formular (firstname, second_firstname, lastname, zipcode, city, street, additionaladdress, country_id_from_apps_countries, username, email, hash, dday, dmonth, dyear, religion, housenumber, activation_token) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)");
    $stmt->bind_param("sssisssssssssssss", $firstname, $secondFirstname, $lastname, $zipcode, $city, $street, $additionaladdress, $country, $username, $email, $hash, $DOB, $MOB, $YOB, $religion, $houseNumber, $activationToken);
    $stmt->execute();
    if ($stmt->affected_rows) {
    }