[ticket/16538] Add identifiers quoting

If reserved word is used as identifier, it must be quoted.
If alias is used elsewhere in SQL statement, it must be quoted.

PHPBB3-16535
PHPBB3-16538

phpBB/phpbb/db/driver/mysqli.php

@@ -489,4 +489,12 @@ class mysqli extends \phpbb\db\driver\mysql_base
 			break;
 		}
 	}
+
+	/**
+	* {@inheritDoc}
+	*/
+	function sql_quote($msg)
+	{
+		return '`' . $msg . '`';
+	}
 }