Fatal error: Unable to connect to database server: Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (111)

/var/www/home_web/web_denif/www/includes/sql.inc.php line 73
          or debug::fatalError('Unable to connect to database server: ' . mysql_error());

debug::fatalError(
'Unable to connect to database server: Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (111)'
)
/var/www/home_web/web_denif/www/shared.inc.php line 64
 sql::connect();

sql::connect()
/var/www/home_web/web_denif/www/index_teachers.php line 5
 require_once 'shared.inc.php';

require_once(
'/var/www/home_web/web_denif/www/shared.inc.php'
)