A PDO connection classto initiate multiple connection and easy access
You have some issues in code design and codestyle:


All class names must be Capitalized
Never use self::$field if $field is protected, use static::$field instead. If you use self child classes won't have access to redefine field!
Never use die() you have exceptions for this!
Save process time use if ($con = database::getConnections())
Class containing only static fields is useless and fully procedural-style, create Singleton or Registry instead.