calling classes

I have a class which extends to include a mysql class. Within the class's functions i have multiple calls (without creating an object like '$instance = new MysqlDb()' ) to the mysql class like this: function doThis(){ $this->query("...."); $this->selectquery(); do something else here.... $this->query("...."); $this->selectquery(); } My question is whether I should beware of two instances of the mysql object getting mixed up somehow. I feel I should clear each instance after use, like I would nullify a variable with $var=""; My scripts seem to run fine, but i wonder if the above is bad practice... Malc