I'm trying to get lines' number from the result of the sql query, so I started by connecting to the database, then checking if the connection was established and finally I tried to count the number of lines, here's the code :

```
 <?php   
    $servername = "localhost";
    $username = "root";
    $password = "";
    $dbname = "db_test";

    // Create connection
    $conn = new mysqli($servername, $username, $password, $dbname);

    // Check connection
    if ($conn->connect_error) {
        die("Connection failed: " . $conn->connect_error);
    }
        $res = $conn->query('SELECT COUNT (id_offre) AS nb FROM offres');

        $data = $res->fetch();

        $nb = $data['nb'];

        echo $nb;
?>

```

I get these errors :

> 
> Uncaught Error: Call to a member function fetch() on boolean
> 
> 
> Call to a member function fetch() on boolean
> 
> 
> 

I used echo to check the 'nb' value, what is the problem?