how to make drupal use mysqlnd_ms
That extension's operations are transparent to the application, you only need to configure the host that the DB connection is setup with.

From a Drupal point of view, you just need to update the "host" entry for the relevant database in settings.php to match the name of your mysqlnd_ms application. From the looks of your config file that's myapp, so your database connection might look like this:

$databases['default']['default'] = array(
  'driver' =&gt; 'mysql',
  'database' =&gt; 'foo',
  'username' =&gt; 'bar',
  'password' =&gt; 'baz',
  'host' =&gt; 'myapp',
  'prefix' =&gt; '',
);


If that doesn't work, check that the extension is installed, configured, and enabled.