Php 请有人解释一下为什么我会收到下面的错误信息？

使用mysqli会更安全、更好

第一个连接：$db=mysqli_连接‘主机’、‘用户’、‘密码’、‘数据库’

接下来，我们将提出我们的问题：

$check_day = "select * from timetable where day ='$day'";
$run = mysql_query($check_day);
if(mysql_num_rows($run) > 0) {
  echo "<script>alert('day $day already exists in our database, please try another one!')</script>";
  exit();
}

$query = "insert into timetable (classes, courses, lecturers, time, room, day) values ('$classes','$courses','$lecturers','$time','$room','$day')";
if (mysql_query($query)) {
  echo "<script>alert('Registration Successful!')</script>";
}
}

现在添加了查询，您可以提供额外的操作：

$query = "insert into timetable (classes, courses, lecturers, time, room, day) values ('$classes','$courses','$lecturers','$time','$room','$day')";
$result = mysqli_query($db, $query);

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您正在使用过时的PHP/MySQL函数

使用新的mysqli_*函数，因为它们更加安全和更新

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我希望这对你有帮助你犯了什么错误？说。即使是这样也不安全！SQL注入！这不再只是早餐了！请已在PHP7中删除。了解有关使用PDO的语句，并考虑使用.@ Felixn，您的代码可以用SQL注入进行黑客攻击，不这样写代码@ JohnConde：保留字是什么？时间和日期是关键字，但不保留。

//Your extra code here

    /* As I read all the previous comments and find out that you are using old SQL query. which is a bad practice. */

    # Note: Please read what is mysqli, why we use mysqli

    # I have correct your query 



 // Create connection
$conn = mysqli_connect($servername, $username, $password, $dbname);
// Check connection
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}


      $check_day = "SELECT * FROM timetable WHERE day ='".$day."'";

    $run = mysqli_query($conn,$check_day);

    if(mysqli_num_rows($run) >0) {

        echo "<script>alert('day $day already exists in our database, please try another one!')</script>";
            exit();
    }else{

    $query = "INSERT INTO timetable (`classes`, `courses`, `lecturers`, `time`, `room`, `day`) VALUES ('".$classes."','".$courses."','".$lecturers."','".$time."','".$room."','".$day."')";

    if(mysqli_query($conn,$query)){
                echo "<script>alert('Registration Successful!')</script>";
    }

    }