Unable to connect to your database server using the provided settings.

Did you change anything in the codeigniter.php file in /system/core?
Because according to the message, if you are using version 3.1.1, line 517 has no relation to databases:

PHP Code:

/* <== line 510
 * ------------------------------------------------------
 *  Instantiate the requested controller
 * ------------------------------------------------------
 */
 // Mark a start point so we can benchmark the controller
 $BM->mark('controller_execution_time_( '.$class.' / '.$method.' )_start');
// <== line 517
 $CI = new $class(); // <== line 518

/*