Ошибка входа в PHP - он всегда позволяет мне войти, независимо от того, существует пользователь или нет

У вас довольно много ошибок/ошибок в коде.

Пусть начнется сверху.

$logsql = mysqli_query("SELECT Name FROM practice.users WHERE Name = $accountname and Password = $password;");

mysqli_query требуется параметр подключения. Использование mysqli_query в запросе с пользовательским вводом также представляет большой риск для безопасности.

Пусть это изменит, используя подготовленные заявления.

$stmt = $connect->prepare("SELECT Name FROM 'practise.users' WHERE Name = ? AND Password = ?");

if(!$stmt) {
    // Something went wrong, create an error with a class to display it accordingly
    $error = [
        'msg' => 'The server could not process your request',
        'class' => 'whatever class you use for error handling',
    ];
}

if(!$stmt->bind_param('ss', $accountname, $password)) {
    // Something went wrong, create an error with a class to display it accordingly
    $error = [
        'msg' => 'The server could not process your request',
        'class' => 'whatever class you use for error handling',
    ];
}

if(!$stmt->execute()) {
    // Something went wrong, create an error with a class to display it accordingly
    $error = [
        'msg' => 'The server could not process your request',
        'class' => 'whatever class you use for error handling',
    ];
} else {
    // Query ran succesfully, lets get the result
    $stmt->store_result();

    $rows = $stmt->num_rows();
}

Судя по тому, что вы также выбираете поле пароля, я подозреваю, что вы храните пароли в текстовом виде. Это еще один риск для безопасности.

Хешируйте пароли так.

password_hash($passwordToHash, PASSWORD_DEFAULT);

И проверьте их как это (не проверяйте введенный пароль перед его проверкой).

password_verify($passwordToValidate, $passwordHash);

Теперь проверьте, существуют ли пользователи и что они вошли в систему.

Вы использовали < 0 который не будет работать, так как он означает меньше нуля.

Это должно сработать.

else {
 // Query ran succesfully, lets get the result
 $stmt->store_result();

 $rows = $stmt->num_rows();

 if($rows > 0) {
     // More than 0 results, the user exists
     session_start();

     // Set a session variable for the username
     $_SESSION['username'] = $accountname;

     // Redirect the user to the secured page (optional)
     header('Location: secured-page.php');
 } else {
     // The result is 0, the user doesn't exist
     $error = [
         'msg' => 'That user does not exists.',
         'class' => 'whatever class you use for error handling',
     ];
 }
}

Общий код для утверждения и проверки

$stmt = $connect->prepare("SELECT Name FROM 'practise.users' WHERE Name = ? AND Password = ?");

if(!$stmt) {
    // Something went wrong, create an error with a class to display it accordingly
    $error = [
        'msg' => 'The server could not process your request',
        'class' => 'whatever class you use for error handling',
    ];
}

if(!$stmt->bind_param('ss', $accountname, $password)) {
    // Something went wrong, create an error with a class to display it accordingly
    $error = [
        'msg' => 'The server could not process your request',
        'class' => 'whatever class you use for error handling',
    ];
}

if(!$stmt->execute()) {
    // Something went wrong, create an error with a class to display it accordingly
    $error = [
        'msg' => 'The server could not process your request',
        'class' => 'whatever class you use for error handling',
    ];
} else {
 // Query ran succesfully, lets get the result
 $stmt->store_result();

 $rows = $stmt->num_rows();

 if($rows > 0) {
     // More than 0 results, the user exists
     session_start();

     // Set a session variable for the username
     $_SESSION['username'] = $accountname;

     // Redirect the user to the secured page (optional)
     header('Location: secured-page.php');
 } else {
     // The result is 0, the user doesn't exist
     $error = [
         'msg' => 'That user does not exists.',
         'class' => 'whatever class you use for error handling',
     ];
 }
}

//wITHOUT TAKING SECURITY INTO CONSIDERATION AND BEST PRACTICESS CONSIDER BELOW

$accountname = $_POST['logname'];
$password = $_POST['logpassword']; 
echo '<br>';//WHY THIS LINE BREAK
//$logsql = mysqli_query("SELECT Name FROM practice.users WHERE Name = $accountname and Password = $password;");

// ADD A CONNECTION BEFORE A THE QUERY SEPERATED BY A COMMA
// ADD SINGLE QUOTES ON '$accountname' AND '$password' VARIABLES
$logsql = mysqli_query($connection, "SELECT Name FROM practice.users WHERE Name = '$accountname' AND Password = '$password'");
if (mysqli_num_rows($logsql) < 0) {
     echo 'Account doesnt exist';
}
else {
    echo 'Welcome ' . $accountname;
}