Warning: mysql_select_db(): supplied argument is not a valid ...

$servername='bla bla'; 
$database_username='bla bla'; 
$database_password='bla bla';
$database_name='bla bla'; 

$link = mysqli_connect($servername,$database_username,$database_password) or die("Erreur de connexion au serveur"); //mysql_connect
mysqli_select_db($link, $database_name) or die("Erreur de connexion à la BDD"); //old: mysql_select_db