Array dereferencing, which is what you are using, was introduced in PHP 5.4 and won’t work in PHP 5.3.
So
$dbSettings = $sm->get( 'Config' )[ 'doctrine' ][ 'connection' ][ 'orm_default' ][ 'params' ];
Would need to be:
$dbSettings = $sm->get( 'Config' );
$params = $dbSettings[ 'doctrine' ][ 'connection' ][ 'orm_default' ][ 'params' ];