Error with PHP and MySQL

Possible errors in your code

• El uso de diferentes Extensiones to manipulate the Connection (mysqli) and to execute the same (mysql)
• No need to Select the database, if you have previously passed as a parameter to the function mysqli_connect

Dejar de Usar la extensión Mysql por problemas de seguridad y además está obsoleta.

The final code would be like this:

require("conexion.php");
require("error.php");
$link= mysqli_connect($dbhost,$userdb,$passworddb,$dbname);

 if ($link->connect_error) {
        die("Connection failed: " . $link->connect_error);
    } 
  $query= "SELECT DISTINCT sk_tipo FROM 'sk_standar' WHERE sk_tipo = 'ALTU-148' UNION SELECT DISTINCT sk_tipo FROM 'sk_standar' WHERE sk_tipo = 'ALTU-160'";
  $result = mysqli_query($link,$query);
  echo '<option value="0">Escoger altura</option>';
  while ($row=mysqli_fetch_array($result,MYSQLI_ASSOC)) {
    echo '<option value="'.$fila["sk_tipo"].'">'.$fila["sk_tipo"].'</option>';
  }
  mysqli_free_result($result);
  mysqli_close($link);
?>

Hello the error that is giving you, is because you are only passing the connection to the mysqli_select_db method; and you are missing the database mysqli_select_db (connection, "bdname"); . Now if you are using mysqli you do not need to call the mysqli_select_db method because the mysqli_connect you are passing the database, I leave you the example of a function that you can use to make your connection to the database Ej:

define("SERVER", "dbserver");
define("USERNAME", "administrador");
define("PASSWD", "1234567");
define("DATABASE", "sk_modular");    

public static function conexion(){
        include_once "config.php";
        $conexion = new mysqli(constant('SERVER'),constant('USERNAME'),constant('PASSWD'),constant('DATABASE'))or die('No se pudo conectar: ' . mysqli_connect_error());
        $conexion->query("SET NAMES 'utf8'");
        return $conexion;
    }

We call the connection and execute the query:

$db = classDb::conexion();    

$query= "SELECT DISTINCT sk_tipo FROM 'sk_standar' WHERE sk_tipo = 'ALTU-148' UNION SELECT DISTINCT sk_tipo FROM 'sk_standar' WHERE sk_tipo = 'ALTU-160'";
      $result = $db->query($query)
      echo '<option value="0">Escoger altura</option>';
      while (($fila = mysqli_fetch_array($result)) != NULL) {
        echo '<option value="'.$fila["sk_tipo"].'">'.$fila["sk_tipo"].'</option>';
      }

      //Cerrar la conexión
      mysqli_close($db);

  

Note: if you are using mysqli you can not combine it with mysql and it's something I'm seeing in your code.

require("conexion.php");
require("error.php");
$link= mysqli_connect($dbhost,$userdb,$passworddb,$dbname);

 if ($link->connect_error) {
        die("Connection failed: " . $link->connect_error);
    } 
   $query= "SELECT DISTINCT sk_tipo FROM 'sk_standar' WHERE sk_tipo = 'ALTU-148' UNION SELECT DISTINCT sk_tipo FROM 'sk_standar' WHERE sk_tipo = 'ALTU-160'";
  $result = mysql_query($query)
            or die("Ocurrio un error en la consulta SQL");
            mysql_close();
  echo '<option value="0">Escoger altura</option>';
  while (($fila = mysql_fetch_array($result)) != NULL) {
    echo '<option value="'.$fila["sk_tipo"].'">'.$fila["sk_tipo"].'</option>';
  }

  //liberar resultador
  mysql_free_result($result);
  //Cerrar la conexión
  mysql_close($link);
?>

I guess you should delete the mysql_select_db() , put in the $ link and you're already passing the database to connect.