php - Constructor vs static instance -

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i new php , had work existing code base. in library there class deals mysql database. however, start instantiate object static getinstance function:

public static function get_instance() {   if(null == self::$instance) {     $c = __class__;     self::$instance = new $c();   }   return self::$instance; }

i don't see advantage of static function in comparison usual constructor. how have deal login credentials? have make them static, too?


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