Mysqli_query返回false，而Mysqli_error返回NULL？

Question

I've been creating a booking system, and creating appointments, but my SQL statement is not working. I've been trying to find a solution but to no avail.

Listed below is my php code. My first SQL statement works perfectly and returns the correct ClientID, however, the second SQL statement does not insert it all into the database. I have done var_dumps on result, returning bool(false), as well as mysqli_error on the result, returning null. My error message at the end only displays the echo'd message, and not the mysqli_error or error number also.

(Note: some values are changed/removed to protect data)

<?php
    session_start();
    if(! $_SESSION['Username']) {
        header("location:Index.php");
    }    
    $servername = "localhost";
    $username = "root";
    $password = "";
    $dbname = "";
    $tablename = "appointmentinformation";
    $tablenamed = "clientinformation";

    $connection = mysqli_connect("$servername", "$username", "$password", "$dbname") or die("Could not connect to the database");

    $clientusername = $_SESSION['Username'];
    $sql = "SELECT ClientID FROM $tablenamed WHERE Username = '$clientusername' LIMIT 1";
    $results = mysqli_query($connection, $sql);
    if (! $results) {
        echo ("Could not select the data : " . mysql_error());
    } else {
        $datarows = mysqli_fetch_row($results);
        $clientid = $datarows[0];
    }

    $date = $_POST["Date"];
    $month = $_POST["Month"];
    $year = $_POST["Year"];
    $time = $_POST["Time"];
    $length = $_POST["Length"];

    $date = stripslashes($date);
    $month = stripslashes($month);
    $year = stripslashes($year);
    $time = stripslashes($time);
    $length = stripslashes($length);

    $date = mysqli_real_escape_string($date);
    $month = mysqli_real_escape_string($month);
    $year = mysqli_real_escape_string($year);
    $time = mysqli_real_escape_string($time);
    $length = mysqli_real_escape_string($length);

    $query = "INSERT INTO appointmentinformation (ClientID, Length, Date, Month, Year, Time, Price) VALUES ('$clientid', '$length', '$date', '$month', '$year', '$time', '$price')";
    $result = mysqli_query($connection, $query);
    if ($result) {
        header("Location:UserCP.php");
    } else {
        echo ("Could not insert data : " . mysqli_error($result) . " " . mysqli_errno());
    }
?>

Answer 1

$query = "INSERT INTO appointmentinformation (ClientID, Length, Date, Month, Year, Time, Price) VALUES ('$clientid', '$length', '$date', '$month', '$year', '$time', '$price')";
    $result = mysqli_query($connection, $query);
    if ($result) {
        header("Location:UserCP.php");
    } else {
        echo ("Could not insert data : " . mysqli_error($result) . " " . mysqli_errno());
    }

The result returned from mysqli_query is null. This sends you down the else branch of the code. Then you coded mysqli_error($result) which equals mysqli_error(null).

The documentation I have read says to initialize a variable with a "link" to the query. You did this with:

 $connection = mysqli_connect("$servername", "$username", "$password", "$dbname") or die("Could not connect to the database");

You now want to code it as mysqli_error($connection) and mysqli_errno($connection).

An additional suggestion. Add this code right after your mysqli_connect statement.

if (mysqli_connect_errno())
  {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
  }