用户24小时按一次按钮?php-mysql

<?php
$username = $_SESSION['username'];  
include('dbcon.php');   
if (isset($_POST['req'])) {

        $query = "SELECT requested FROM users WHERE username='$username' ";
        $result = mysqli_query($db, $query);
        $row = mysqli_fetch_array($result);
        if(empty($row['requested'])) {

    $request = mysqli_real_escape_string($db, $_POST['request']);
    $query = "INSERT INTO requests (username, airid) 
              VALUES('$username', '$request')";

    mysqli_query($db, $query);

    $query2 = "UPDATE users SET requested='1' WHERE username='$username'"; 
    mysqli_query($db, $query2); 
    // timer should go here i guess

    echo '<script>swal({title: "Great!", text: "We will process your request as soon as possible!", type: "success", buttonsStyling: false, heightAuto: false}).then(function(){ location.reload();});</script';    

        } else { 

            echo '<script>swal({title: "Whoa!", text: "You must wait 48 hours before requesting again!", type: "error", buttonsStyling: false, heightAuto: false}).then(function(){ location.reload();});</script'; 
}
}
?>  

如何在将users requested=设置为“1”后设置计时器,以便在24小时后,他的请求将不再是“1”,而是再次为“空”

对不起我的蹩脚英语