Error php sql: Call to a member function bind_param() on boolean

When I tried to run this function I get:

Fatal error: Uncaught Error: Call to a member function bind_param() on boolean in /var/www/html/include/function/token.php:73 Stack trace: #0 /var/www/html/view.php(14): dirTOperm('/var/www/html/u...') #1 {main} thrown in /var/www/html/include/function/token.php on line 73

The function is as follows:

function dirTOperm($dir){
    global $mysql_ip, $mysql_user, $mysql_pass, $mysql_database_name,$_SESSION;


    $conn = new mysqli($mysql_ip, $mysql_user, $mysql_pass, $mysql_database_name);
    $stmt = $conn->prepare('SELECT * FROM token-perm WHERE directory = ?');
    $stmt->bind_param('s', $dir);
    $stmt->execute();
    $result = $stmt->get_result();
    $checktoken = $result->num_rows;
    $row = $result->fetch_assoc();
    $stmt->close();
    $conn->close();

    if ($checktoken == 0) {
      $permtoken = GenerateRandomString(512);

      $conn = new mysqli($mysql_ip, $mysql_user, $mysql_pass, $mysql_database_name);
      $stmt = $conn->prepare('INSERT INTO token-perm (token,directory,date) VALUES ("?","?","?")');
      $stmt->bind_param('sss', $permtoken,$dir,date('d-m-Y H:m'));
      $stmt->execute();
      $stmt->close();
      $conn->close();
      return $permtoken;
    }else {
      return $row['token'];
    }
  }

Connection data is correct and trying to search on google I have not risen to solve, can you help me?