CodeIgniter: Unable to connect to your database server using the provided settings Error Message

I think, there is something wrong with PHP configration.
First, debug your database connection using this script at the end of ./config/database.php :

...
  ...
  ...
  echo '<pre>';
  print_r($db['default']);
  echo '</pre>';

  echo 'Connecting to database: ' .$db['default']['database'];
  $dbh=mysql_connect
  (
    $db['default']['hostname'],
    $db['default']['username'],
    $db['default']['password'])
    or die('Cannot connect to the database because: ' . mysql_error());
    mysql_select_db ($db['default']['database']);

    echo '<br />   Connected OK:'  ;
    die( 'file: ' .__FILE__ . ' Line: ' .__LINE__); 

Then see what the problem is.

Today I fallen this kind of problem in live server and i solved the problem changing this line

$db['default']['db_debug'] = TRUE;

to

$db['default']['db_debug'] = FALSE;

For me the issue was in the php.ini file. The property mysql.default_socket was pointing to file in a non-existent directory. The property was pointing to /var/mysql/mysql.sock but in OSX, the file was located in /tmp/mysql.sock.

Once I updated the entry in php.ini and restarted the webserver, the issue was resolved.